Directions:
1. Cupcakes: Preheat your oven to 350°F (175°C). In a large bowl, whisk together the flour, sugar, baking soda, and baking powder. Add the softened butter, egg whites, vanilla extract, sour cream, and milk; beat until the batter is smooth and creamy. Fill cupcake liners about 2/3 full and bake for 18-20 minutes or until a toothpick inserted comes out clean. Allow to cool completely on a wire rack.
2. Filling: In a medium saucepan, heat the heavy cream until it starts to simmer. In a separate bowl, whisk together the egg yolks, sugar, and cornstarch until smooth. Gradually temper in the hot cream by slowly whisking it into the egg yolk mixture to avoid cooking the eggs. Return the mixture to the saucepan and heat, constantly stirring, until the mixture thickens to a custard-like consistency. Remove from heat and stir in the vanilla extract. Transfer to a bowl, cover with plastic wrap directly on the surface to prevent a skin from forming, and chill in the refrigerator until cold.
