Preheat oven to 300°F. Grease an 8 inch round cake pan, and line with parchment paper. Grease the parchment paper and dust the pan with cocoa powder

In the top of a double boiler over lightly simmering water, melt chocolate and butter. (Or for the lazy version, but the chocolate and butter into a microwave-safe bowl and microwave for 30 second increments, stirring in between, until both are melted.)

Remove chocolate and butter from heat and stir in sugar, cocoa powder, eggs, and vanilla. Pour into prepared pan.

Bake in preheated oven for 30 minutes or until a toothpick inserted in the center comes out clean. Let cool in pan for 10 minutes, then turn out onto a wire rack and cool completely.