Coat two 9x1-1/2-inch round baking pans with nonstick cooking spray.
In a heavy saucepan, melt butter. Add beer and cocoa powder; stir until well combined. Remove from heat and let mixture cool slightly.
Meanwhile, in a large bowl, stir together flour, sugar, baking soda, and salt. Add the cocoa-beer mixture and beat with an electric mixer on medium speed for 1 minute. Add sour cream and beat for 1 minute. Add eggs; beat for 1 minute or until thoroughly blended. Pour the chocolate cake batter into the prepared pans.
Bake in a 350 degree F oven about 30 minutes or until a toothpick inserted near the center of each cake layer comes out clean. Cool in pans on wire racks for 10 minutes. Remove from pans; cool thoroughly on wire racks.
Place a cake layer on a serving plate. Spread some of the Chocolate Ganache Frosting on top. Place second layer on top. Frost top and side of cake with remaining frosting. To make top smooth, dip a long metal spatula into hot water until the spatula is hot, then pull the spatula across the top of the cake.
Decorate the cake by drizzling it with the melted white baking bar. Serve immediately or cover and refrigerate the cake until serving time. Remove the cake from the refrigerator 30 minutes before serving to allow the chocolate to soften. Refrigerate leftovers. Makes 10 to 12 servings.
Heat whipping cream just to boiling. Meanwhile, finely chop semisweet or bittersweet chocolate and place in a stainless steel bowl. Pour hot cream over chocolate and stir with a rubber spatula until chocolate is completely melted with no hardened specks remaining. Place bowl in a larger bowl filled with ice. Stir with spatula until mixture firms up with no lumps. Stop stirring after about 8 minutes, just before you think the mixture is spreadable. Take bowl out of ice and let the chocolate frosting stand for 1 minute before using.