Inspired by the old-school, ultra-rich, mousselike chocolate cake that usually called for a whole pound of chocolate, half a dozen eggs and lots of butter, this enlightened rendition has deep bittersweet chocolate flavor and dense melt-in-your-mouth texture. No one will guess it's healthier. The secret is excellent natural cocoa powder and good-quality bittersweet chocolate, preferably with 70% cacao. Although the cake can be eaten once it's completely cool, it comes out of the pan much easier and even tastes better if it has been chilled at least overnight.