Heat oven to 300. Spray a 9" round or square baking pan w/ cooking spray. Dust pan w/ a little cocoa powder. In microwaveable bowl, melt chocolate and butter in 30 second intervals, stirring often, until melted.
Stir in sugar, cocoa powder, eggs and vanilla.
Pour into pan. Bake about 30 mins, or until a toothpick inserted in center comes out clean.
Cool. Invert pan onto plate. Cool. Slice and serve w/ a dollop of whipped cream.