Preheat oven to 300. Grease an 8 inch round pan, and dust with cocoa powder. Melt chocolate in double broiler or heavy bowl over hot water, melt chocolate and butter. Remove from heat, stir in sugar, cocoa powder, eggs and vanilla. Pour in pan. Bake in oven for 30 min. Let cool in pan 10 min. Remove and cool on rack or cake board.