Preheat the oven to 350°F (176°C) and line a cupcake pan with cupcake liners.
In a large mixing bowl, cream together the butter and sugar until light and fluffy, about 3 to 4 minutes.
Add sour cream and vanilla extract to the butter mixture and mix until well combined.
Incorporate egg whites in two batches, mixing well after each addition.
In a separate bowl, combine all-purpose flour, baking powder, and salt.
Mix milk and water in a small measuring cup.
Gradually add half of the dry ingredients to the wet batter, mixing well.
Add the milk mixture and mix until incorporated. Then add the remaining dry ingredients.
Fill the prepared cupcake liners halfway with batter.
Bake for 15 to 17 minutes, or until a toothpick comes out with a few crumbs.
Remove from oven and let cool for 2 to 3 minutes in the pan before transferring to a cooling rack.
In a medium bowl, beat the egg yolks and set aside.
In a large saucepan, combine sugar, cornstarch, and milk, mixing until smooth and cooking over medium heat.
Stir continuously until the mixture thickens and bubbles.
Tempering the egg yolks by whisking in some of the hot milk mixture and returning it to the saucepan.
Cook the mixture over medium heat until it comes to a light boil, then cook for an additional 2 minutes.
Stir in butter and vanilla until smooth and set aside to cool.
In a large bowl, combine the chocolate chips and corn syrup.
Bring the heavy cream to a boil and pour over the chocolate mixture. Let sit for 3 to 5 minutes before whisking until smooth.
Let the ganache cool in the fridge until it firms up.
Cut out the centers of cooled cupcakes and fill each with pastry cream.
Top with chocolate ganache using a piping bag for decoration.
Refrigerate until ready to serve, preferably within 3 to 4 days.