Preheat oven to 350°F (176°C) and prepare a cupcake pan with cupcake liners.

In a large mixing bowl, cream butter and sugar together until light in color and fluffy, about 3-4 minutes.

Add sour cream and vanilla extract and mix until well combined.

Add egg whites in two batches, mixing until well combined after each.

Combine dry ingredients in a separate bowl, then mix milk and water.

Add half of the dry ingredients to the batter and mix well, then add the milk mixture, and mix well. Finally add remaining dry ingredients and mix until combined.

Fill the cupcake liners about halfway. Bake for 15-17 minutes.

Remove from oven and cool for 2-3 minutes before transferring to cooling rack.

While cupcakes cool, make pastry cream. Beat egg yolks in a medium bowl, set aside.

Mix sugar, cornstarch, and milk in a large saucepan until smooth.

Cook over medium-high heat, stirring until mixture thickens and bubbles.

Reduce heat to medium and simmer for 2 minutes, then remove from heat.

Add a little milk mixture to the egg yolks to temper, then combine back into the milk mixture.

Bring back to a boil and cook for another 2 minutes, stirring continuously.

Remove from heat and add butter and vanilla extract, stir until smooth.

While cream cools, prepare chocolate ganache by combining chocolate chips and corn syrup in a bowl.

Bring heavy cream to a boil and pour over chocolate chips. Let sit for 3-5 minutes, then whisk until smooth.

Chill ganache in the fridge until firm, 1-2 hours.

Once cool, cut out centers of the cupcakes.

Fill cupcakes with pastry cream.

Top with chocolate ganache.

Refrigerate until ready to serve.