
Preheat oven to 300º F. Grease an 8" round cake pan, and dust with cocoa powder.
Melt the chocolate and butter, two options: In the top of a double boiler over lightly simmering water, melt chocolate and butter. OR microwave in a microwave-safe bowl in 30-second bursts, being careful not to over-nuke.
Remove from heat, and stir in sugar, cocoa powder, eggs, and vanilla. Pour into prepared pan.
Bake in preheated oven for 30 minutes. Let cool in pan for 10 minutes, then turn out onto a wire rack and cool completely. Slices can be reheated for 20 to 30 seconds in the microwave before serving.