ConsumerLab's tests of popular brands of unsweetened cocoa powder show that a tablespoon (5 grams) provides from 37 to 130 milligrams of cocoa flavanols, depending on the brand. Cocoa flavanols are key compounds associated with cocoa's health benefits.
ConsumerLab's tests of popular sweetened dark chocolates found that a 40 gram serving (the size of a medium bar) provides from 136 mg to 440 mg of flavanols depending on the brand. The same amount of unsweetened baking chocolate provides a whopping 592 mg of flavanols.
As you can see, a tablespoon of cocoa powder rich in flavanols will provide about as many milligrams of flavanols as an entire bar of the weakest dark chocolate or just one-quarter of a bar that is rich in flavanols. It all depends on the product you choose. A benefit of choosing a dark chocolate rich in flavanols is that you'll get those flavanols while minimizing calories. Unfortunately, you won't be able determine flavanol content from reading labels because flavanols are rarely listed and even the "% cacao" shown on some labels is not a reliable indicator.
You can find out the flavanol (and calorie) content of cocoa powders and dark chocolates tested by ConsumerLab.com in the Cocoa Powders, Dark Chocolate, Extracts, Nibs, & Supplements Review >>