For vanilla cream, in a small bowl lightly beat egg yolks; set aside. In a medium saucepan combine sugar and cornstarch. Whisk in milk. Cook and stir over medium heat until thickened and bubbly. Cook and stir for 2 minutes more.
Gradually stir about 1 cup of the hot mixture into the egg yolks, stirring constantly. Return all the mixture to the saucepan. Cook and stir until bubbly. Cook and stir for 2 minutes more. Remove from heat. Stir in butter until melted. Stir in vanilla. Cover surface with plastic wrap. Chill in the refrigerator for at least 2 hours or up to 24 hours.
To assemble, using a serrated knife, cut each vanilla cupcake horizontally in half. Spoon 1 heaping tablespoon of vanilla cream onto the cut side of each cupcake bottom. Replace cupcake tops. Place cupcakes on a wire rack set over waxed paper.
For ganache, in a medium microwave-safe bowl microwave chocolate pieces and whipping cream on 100 percent power (high) for 1 to 2 minutes or until chocolate is almost melted. Stir until smooth.
Spoon ganache over cupcakes. Let stand until chocolate is set. Chill in the refrigerator for at least 1 hour before serving. Makes 12 (2-1/2 inch) cupcakes.