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. Do not skimp on the amount of creaming time. Add sour cream and vanilla extract and mix until well combined. Add egg whites in two batches, mixing until well combined after each. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated.

Combine dry ingredients in a separate bowl. In small measuring cup, combine the milk and water. Add half of the dry ingredients to the batter and mix until well combined. Add the milk mixture and mix until well combined. Add remaining dry ingredients and mix until well combined. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated.

Fill the cupcake liners about halfway. Bake for 15-17 minutes, or until a toothpick inserted comes out with a few crumbs. Remove cupcakes from oven and allow to cool for 2-3 minutes, then remove to cooling rack to finish cooling.

PASTRY CREAM: Put yolks in a medium bowl and gently beat. Set aside.

In saucepan add the sugar, cornstarch and milk and mix until smooth. Cook, stirring continuously, over medium-high heat until mixture begins to thicken and bubble. Reduce heat to medium and simmer for 2 minutes. Remove from heat.

Add a little bit of milk mixture to the egg yolks and whisk together, then add egg mixture to milk mixture. This process ensures that you dont cook the egg yolks. Place the pan back on the heat and bring to a light boil. Allow to boil for 2 minutes, stirring continuously. Remove from heat and add butter and vanilla extract. Stir until smooth, then set aside to cool to room temperature.

GANACHE: While the cupcakes and cream are cooling, make the chocolate ganache. Put the chocolate chips and corn syrup in a large bowl.

Bring cream to a boil, then pour over the chocolate. Sit for 3-5 minutes, then whisk until smooth.Place in the fridge until completely cool and firm, 1-2 hours.

ASSEMBLY: Once everything is cooled and ready, cut out the centers of the cupcakes. I use a cupcake corer, but a knife would work as well. Divide the pastry cream between the cupcakes and fill in the centers.

Top with the chocolate ganache. Refrigerate cupcakes until ready to serve. Cupcakes are best for 2-3 days.