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 combine the milk and water in a small measuring cup.

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.

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

Remove cupcakes from oven and allow to cool for 2-3 minutes, then remove to cooling rack.

While the cupcakes cool, make the pastry cream. Put the egg yolks in a medium sized bowl and gently beat them together.

Add the sugar, cornstarch and milk to a large saucepan and mix until smooth.

Cook over medium-high heat until mixture begins to thicken and bubble.

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

Add some milk mixture to the egg yolks and whisk together, then add egg mixture to milk mixture.

Place the pan back on the heat and bring to a light boil, allow to boil for 2 minutes.

Remove from heat and add butter and vanilla extract. Stir until smooth.

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

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

Place the chocolate ganache in the fridge until completely cool and firm, 1-2 hours.

Once everything is cooled, cut out the centers of the cupcakes.

Divide the pastry cream between the cupcakes and fill in the centers.

Top the cupcakes with the chocolate ganache.

Refrigerate cupcakes until ready to serve and serve closer to room temperature.