01 -
Mix sugar, cornstarch, and milk in a large saucepan until smooth. Cook over medium heat, stirring continuously, until thickened and boiling. Reduce heat and simmer for 1 minute, then remove from heat.
02 -
Whisk a small amount of the hot mixture into the egg yolks. Combine egg mixture with the remaining milk mixture and return saucepan to heat.
03 -
Bring the mixture to a light boil, stirring continuously for 2 minutes. Remove from heat, then stir in butter and vanilla until smooth. Allow to cool to room temperature.
04 -
Preheat oven to 350°F. Line two 12-count cupcake tins with liners. Whisk together cake mix, water, melted butter, and eggs until smooth.
05 -
Fill each liner halfway with batter and bake for 15 minutes, or until a toothpick inserted in the center comes out clean. Cool completely on a rack.
06 -
Combine chocolate chips and heavy cream in a microwave-safe bowl. Microwave in 15-second intervals, stirring, until smooth. Sift in powdered sugar and let cool for 10 minutes.
07 -
Cut 1-inch wide and deep centers in cupcakes. Fill with pastry cream using a spoon or piping bag. Top with cooled ganache.