Milk chocolate with a low cocoa content doesn’t offer the goodness that higher-cocoa content chocolate does. The darker the better.
That said, cocoa nibs (the bean bits left over after winnowing) offer the highest amount of antioxidants, fiber, and magnesium, without the extra sugar. Chocolate contains more of the antioxidant, catechins, than green tea, as well as the amino acid tryptophan along with small amounts of phenylethylamine and theobromine -- all mood-boosters. Cocoa nibs can be expensive, and they are exotic (all chocolate pretty much is), but they offer a great way to mix up the chocolate a bit.
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