Preheat oven to 350°F (176°C) and line cupcake pan with liners.
Cream butter and sugar until light and fluffy.
Mix in sour cream and vanilla extract.
Add egg whites gradually, mixing well after each addition.
Combine dry ingredients; mix milk and water in a separate cup.
Alternate adding dry ingredients and milk mixture to batter, mixing well.
Fill cupcake liners halfway and bake for 15-17 minutes.
Cool cupcakes on rack; prepare pastry cream by cooking ingredients until thickened.
Fill cupcakes with cooled pastry cream.
Make ganache by melting chocolate with cream; chill until firm.
Core cupcakes; fill centers with pastry cream.
Top with ganache, refrigerate until serving, and enjoy within 3-4 days.