Preheat oven to 300 degrees F (150 degrees C). Grease an 8 inch round cake pan, and dust with cocoa powder.
In the top of a double boiler over lightly simmering water, melt chocolate and butter. 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 also be reheated for 20 to 30 seconds in the microwave before serving.