Preheat oven to 350°F. Butter 10-inch-diameter springform pan. Line bottom with waxed paper. Stir butter and chocolate in heavy large saucepan over low heat until melted. Add sugar and cocoa and whisk to combine. Add eggs; whisking until well blended. Pour batter into prepared pan. Bake until tester inserted into center comes out clean, about 45 minutes. Cool cake completely in pan on rack. Run knife around pan sides to loosen cake. Release pan sides.

Stir sugar and water in small medium saucepan over low heat until sugar dissolves. Increase heat; boil without stirring until syrup is deep amber color, about 7 minutes. Remove from heat. Add in cream (mixture will bubble vigorously). Return to low heat; stir until any bits of caramel dissolve. Add butter; whisk until smooth.

Drizzle the cake with warm caramel sauce and pile up with scoops of vanilla ice cream and throw some sparklers on there because WHY NOT!