Prepare the cake mix according to manufacturers directions. Pour the batter into a greased 9 by 12-inch cake pan and bake according to package instructions. Remove the cake from the oven and refrigerate for 30 minutes.
With an electric mixer in a large bowl, beat the butter on medium speed for 5 minutes, then add the sugar, chocolate and vanilla extract. Continue to mix for 2 to 3 minutes, stopping during the mixing to scrape down the sides of the bowl. Remove the cake from the oven and apply the chocolate mixture to the top of the cake with a spatula. It should be 1/2 to 3/4-inch thick. Return the cake to the refrigerator and allow the topping to set. Remove the cake from the pan with spatula and cut into 3 by 1-inch fingers with a sharp knife. Arrange on a serving tray and dust with powdered sugar.