Hershey's Special Dark is the chocolate of choice in this recipe. Other bittersweet chocolates will work, but because amounts of sugar and cocoa butter differ from brand to brand, they will produce cakes with slightly different textures and flavors. When crumbling the brown sugar to remove lumps, make sure that your fingers are clean and grease-free; any residual fat from butter or chocolate might hinder the whipping of the whites. If you like, dust the cake with confectioners' sugar just before serving or top slices with a dollop of lightly sweetened whipped cream. Chocolate-Orange variation as well.