Grease an 8-inch or 9-inch round cake pan with butter and dust with cocoa powder.
In a medium saucepan over low heat, melt butter. Remove from heat and stir in cocoa powder, sugar, vanilla, salt and cayenne. Then stir in eggs.
Pour into pan and bake for 30-40 minutes. Let cool in the pan for 10 minutes and finish cooling on a wire rack. Garnish with powdered sugar when ready to serve.