01 - Preheat oven and prepare the yellow cake mix according to the package instructions for cupcakes. Bake as directed and allow cupcakes to cool completely on a wire rack.
02 - In a medium saucepan, whisk together whole milk, granulated sugar, cornstarch, and egg yolks. Cook over medium heat, whisking constantly, until the mixture thickens and bubbles. Remove from heat, add butter and vanilla extract, and stir until smooth. Cover with plastic wrap directly touching the surface and refrigerate until well chilled.
03 - After cupcakes have cooled, use a paring knife or cupcake corer to remove a small portion from the center of each. Fill each cavity with chilled pastry cream using a piping bag or spoon.
04 - In a small saucepan, heat heavy cream over medium heat until simmering. Remove from heat and pour over chopped semi-sweet chocolate in a bowl. Let stand for 2 minutes, then stir until ganache is smooth and glossy.
05 - Spoon prepared ganache over the tops of filled cupcakes. Allow ganache to set slightly before serving.