Preheat your oven to 350°F (175°C) and prepare a standard cupcake pan with liners.

Cream together ½ cup unsalted butter and 1 cup granulated sugar for 3-4 minutes until light and fluffy.

Add ½ cup sour cream and 2 teaspoons vanilla extract, mix in 4 egg whites until blended.

Whisk together 1 ½ cups all-purpose flour, 2 teaspoons baking powder, and ½ teaspoon salt in a separate bowl.

Gradually add the dry ingredients to the wet mixture, alternating with ½ cup milk.

Fill each liner about halfway and bake for 15-17 minutes until a toothpick comes out clean.

Cool in the pan for 5 minutes before transferring to a wire rack to cool completely.

Whisk together ½ cup sugar, ¼ cup cornstarch, and 2 cups milk over medium heat until thickened, about 5 minutes.

Temper 4 egg yolks with a small amount of hot mixture then return to saucepan, boiling for 1 minute.

Stir in 2 tablespoons of butter and 1 teaspoon of vanilla extract, transfer to a bowl to cool.

Heat ½ cup heavy cream until simmering, then pour over 1 cup chocolate chips and whisk until smooth.

Add 1 tablespoon corn syrup and chill ganache until spreadable, around 15-20 minutes.

Remove centers of cooled cupcakes and fill with pastry cream generously.

Spread or pipe the cooled ganache over each cupcake to drape down sides.

Refrigerate for about 30 minutes to set ganache and firm pastry cream.