1. For ganache: In a small saucepan, heat whipping cream over medium-high heat just until boiling. Remove from heat. Add the 8 ounces milk chocolate (do not stir). Let stand for 5 minutes. Stir until smooth. Transfer to a large bowl. Cover and chill.
2. Preheat oven to 350 degrees F. Coat two 9x1-1/2-inch round cake pans with nonstick spray for baking; set aside. Prepare brownie batter according to package directions. Divide batter between prepared pans. Bake about 25 minutes or until edges are set. Cool in pans on wire racks for 10 minutes. Remove uncut brownies from pans. Transfer to a wire rack. Cool completely.
3. Preheat oven to 350 degrees F. In a large bowl, combine cream cheese, sugar, and vanilla; beat with an electric mixer on medium speed until combined. Beat in eggs just until combined. Transfer half of the mixture (about 1-1/2 cups) to a medium bowl. To one bowl, add the 3 ounces melted milk chocolate; stir until smooth. To the other bowl, add the melted white chocolate; stir until smooth.
4. Wrap heavy-duty foil around the bottom of an 8-inch springform pan, making sure foil comes at least 1 inch up the side. Using a sharp knife, trim edges from brownie rounds, making two 8-inch rounds.* Place one of the brownie rounds in the pan. Pour milk chocolate-cream cheese mixture on top. Place the remaining brownie round on the milk chocolate-cream cheese mixture and pour white chocolate-cream cheese mixture on top.
5. Place the foil-wrapped springform pan in a large roasting pan; pour enough hot water around the springform pan to come halfway up the side. Bake for 60 to 65 minutes or until cheesecake appears set when gently shaken. Carefully remove springform pan from water.
6. Cool in springform pan on a wire rack for 15 minutes. Using a small sharp knife, loosen the edge from the side of the pan; cool for 30 minutes more. Remove the side of the pan; cool completely on wire rack. Cover and chill overnight.
7. Remove ganache from refrigerator. Beat with an electric mixer on medium speed until light, fluffy, and of spreading consistency. Spoon whipped ganache into a pastry bag fitted with a medium star or round tip. Pipe stars or mounds to cover the entire top of the chilled cheesecake. Makes 12-16 servings.
*Test Kitchen Tip: Before wrapping the springform pan in foil, place the bottom of the pan on each brownie round and use the tip of a sharp knife to trace around the pan bottom. Trim edges. This will assure the brownie rounds fit tightly in the pan.