We all know that anything that tastes or feels good is bad for you, right? Wrong! We've already covered how Sex Is Good For Males. Well, dark chocolate is yet another example.
Dark chocolate is relatively low on calories and high on flavanoid-rich cocoa. Dove is the gold standard, but almost any true dark chocolate will do.
The merits of dark chocolate are coming out in the studies almost every month, but here's a few of the most important:
1) Heart Health. Cocoa, the main ingredient in dark chocolate, is very good for the heart. It lowers blood pressure among other things. See #4 here.
2) Kuna. Imagine a world with no heart disease. Well, the Kuna live in such a world and cocoa is one of their secret weapons. Read here to find out about the Kuna and the unique lessons we can live from this third world people.
3) Nitric Oxide. Dark chocolate increases Nitric Oxide, the stuff of erections. Yes, it increases blood flow into the penis and that's just what you need for a healthy sex life. For other examples of foods that do the same, consider the Peak Testosterone Erectile Strength Diet.
4) Inflammation. A 2008 study showed that small amounts, one or two squares, of dark chocolate reduced inflammation, which is implicated in heart disease and a host of other ailments.  See my link on Inflammation for more details.
5) Delay In Dementia and Alzheimers. The polyphenols in dark chocolate will very likely delay and prevent Alzheimers and dementia according to one recent study. 
6) Appetite Suppression. Dark chocolate is highly satisfying to the appetite and can actually help you to lose weight. Researchers in Copenhagen showed that men who first consumed dark chocolate later consumed 15% less pizza, which adds up to a lot of calories.  In other words, dark chocolate can actually prevent you from overstuffing the ol' pie hole.
7) Stearic Acid. The primary fat in chocolate is a saturated fat, but has much less negative effects on your blood lipids than most other kinds of saturated fat.
8) Stress Reduction. Well, this is still more good news for chocolate lovers. All you type A's out there, listen to this: Nestle's lab found that in high anxiety subjects, a small bar of dark chocolate significantly reduced stress hormones. 
9) Skin. Consumption of 2 tablespoons of high-flavanoid cacao for 12 weeks resulted in skin that was smoother and more moist, which is important for a more youthful look. 
10) Heart Failure Protection. One 2010 study (of women) showed substantial protection against heart failure hospitalization or death in those who ate just an ounce of chocolate - not necessarily dark in this case - once or twice per week.  Note that eating over one serving per day was associated in a loss of all protective benefits.
11) Brain and Vision. High flavanol dark chocolate was found to significantly increase short term cognitive and visual brain functions.  For example, it improved choice reaction time, spatial memory, visual contrast sensitivity and motion detection for starters. In other words, chocolate is not just a sex booster - it's a brain booster.
12) Medicine. Yes, researchers have actually suggested that dark chocolate could be used as medicine! As I always say, "Food is clinical" and dark chocolate is a perfect example of this. Researchers studied people with high blood pressure (hypertension) and Metabolic Syndrome and found that dark chocolate actually saved lives and decreased mortality.  The researchers emphasized that it needs to be 1) dark chocolate with 2) at least 60-70% cocoa. 
13) Brain Booster. The studies keep rolling in showing that cocoa is a brain builder. For example, a 2012 study in the journal Hypertension, shows that older people with mild cognitive impairment were significantly helped by consuming high-flavonol cocao.  It is important to note that this study showed the benefit ONLY occurring with the higher flanonol cocoas, so your dark chocolate must be of high quality and low processing.
14) Insulin Sensitivity. One of the huge battles for most men (over the age of about 40) is managing their blood glucose and insulin levels. Of course, prediabetes and often Metabolic Syndrome occur when men lose their insulin sensitivity. (This can be triggered by Inflammation.) Again, high quality cocoa can come to the rescue as one study found that it increased insulin sensitivity.  Is there anything this wonder plant can't do???
"What are you waiting for?" That's my only question for those of you who don't eat dark chocolate. It is simply one of those things in life that is too good to be true but really is.
15) Serotonin. Chocolate also contains significant amounts of both serotonin and tryptophan, which are important in the production of serotonin. They believe this explains the calming effect of chocolate on many people. (Magnesium actually helps tryptophan into serotonin.)
16) De-Stiffens Arteries. A recent study found that dark chocolate de-hardened arteries in overweight, middle-aged men. And they also found that it kept white blood cells from sticking to arterial walls, which should help with arteriosclerosis prevention. 
CAUTION: You may want to read my link on Does Chocolate Have An Ideal Dosage? There are some strong indications that there is such a thing as too much chocolate.
CAUTION 2: You may want to read my link on TNF Inhibitors as well. Cocoa may stimulate TNF-alpha production in some cases and this inflammatory cytokine is a root cause for diabetes, arthritis, arteriosclerosis and many other ills. However, it should be pointed out that The Kuna, who are heavy consumers of cocoa, show no ill effects.
CAUTION 3: The only "side effect" that I know related to dark chocolate is that it can relax the valve between stomach and esophagus in some sensitive people, leading to heart burn. Also, if you happen to be the type that actually knows what to do in the kitchen and are using cocoa in your recipes, you will generally want to avoid Dutch-processed cocoa or cocoa "processed with alkali". Alkali processing is used to lower the naturally acidic pH of raw cocoa but, unfortunately, strips out most of the flavanol content. One study, for example, showed that alkali processing lowered the flavanol content from an average of 34.6 to 13.8 and 7.8, respectively, for lightly and heavily processed cocoas.  My understanding is that the standard Hershey's and Nestle's cocoa that you buy in the supermarkets (in the U.S.) use a processing called Broma that leaves the flavanol content largely untouched.
1) J Nutr, 2008;138:1939-1945, "Regular consumption of dark chocolate is associated with low serum concentrations of C-reactive protein in a healthy italian population"
2) J of Alzheimer s Disease, 18(4), in-press, A Diet Enriched in Polyphenols and Polyunsaturated Fatty Acids, LMN Diet, Induces Neurogenesis in the Subventricular Zone and Hippocampus of Adult Mouse Brain.
3) J of Proteome Res, 2009, Published online ahead of print Oct 2009, "Metabolic Effects of Dark Chocolate Consumption on Energy, Gut Microbiota, and Stress-Related Metabolism in Free-Living Subjects"
4) J Nutr, Jun 2006, 136(6):1565-9, "Long-term ingestion of high flavanol cocoa provides photoprotection against UV-induced erythema and improves skin condition in women"
5) University of Copenhagen. "Dark Chocolate Is More Filling Than Milk Chocolate And Lessens Cravings." ScienceDaily 23 December 2008. 22 March 2010
6) Journal Agric Food Chem, 2008, 56 (18):8527 8533, "Impact of Alkalization on the Antioxidant and Flavanol Content of Commercial Cocoa Powders"
7) Circulation: Heart Failure, 2010; 3:612-616, "Chocolate Intake and Incidence of Heart Failure: A Population-Based Prospective Study of Middle-Aged and Elderly Women"
8) Physiol Behav. 2011 Feb 12;103(3-4):255-260, "Consumption of cocoa flavanols results in an acute improvement in visual and cognitive functions"
9) BMJ, 2012, 344: e3657, Published online 2012 May 31, "The effectiveness and cost effectiveness of dark chocolate consumption as prevention therapy in people at high risk of cardiovascular disease: best case scenario analysis using a Markov model"
11) HYPERTENSION, AHA, Published online before print August 14, 2012, "Benefits in Cognitive Function, Blood Pressure, and Insulin Resistance Through Cocoa Flavanol Consumption in Elderly Subjects With Mild Cognitive Impairment"
12) Am J Clin Nutr, 2005 Mar, 81(3):611-4, Short-term administration of dark chocolate is followed by a significant increase in insulin sensitivity and a decrease in blood pressure in healthy persons"
13) http://www.eurekalert.org/pub_releases/2014-02/foas-wdc022714.php, "Why dark chocolate is good for your heart New research in the FASEB Journal suggests that consumption of dark chocolate lowers the augmentation index, a key vascular health predictor, and reduces adhesion of white blood cells to the vessel wall"