Step 1

Preheat the oven to 350* F. Line a cupcake pan with cupcake liners or silicone baking cups.

Step 2

In a stand mixer, cream the butter and sugar together until light and fluffy, around 3-4 minutes. Add the sour cream and vanilla and mix well.

Step 3

Add the egg whites in two batches, mixing well in between each addition. Scrape down the sides of the bowl if needed.

Step 4

In a second bowl, combine the flour, baking powder and salt. In a measuring cup, combine the milk and water. Add half of the dry ingredients to the batter, and mix on low until combined. Add the milk mixture, mix until well incorporated, and then add in the rest of the dry ingredients, mixing until combined. Scrape down the sides of the bowl if you need to, to make sure everything is mixed in well.

Step 5

Fill the prepared cupcake liners 1/2-3/4 full. Bake at 350*F for 15-18 minutes, until the tops are puffed and a cake tester inserted in the cupcake come out clean. Remove the cupcakes from the oven. Allow to cool in the pan for 5 minutes, then remove to a wire rack to finish cooling.

Step 6

Put the egg yolks in a medium sized bowl and mix them up a bit. Set them aside.

Step 7

Place the sugar, cornstarch and milk in a medium sized saucepan, and mix with a whisk until smooth. Place the pan over medium-high heat on the stovetop and cook, stirring continuously, until the mixture begins to bubble. Reduce the heat and simmer for 2 minutes, stirring occasionally. Remove from the heat.

Step 8

Add about 1/4 cup of the milk mixture slowly to the egg yolks and whisk together. Then, add the new egg mixture into the pan with the rest of the custard, and whisk until smooth. (This ensures the eggs won't scramble in the custard.)

Step 9

Place the pan back on the heat and bring to a light boil. Stir continuously, and cook for 2 more minutes until thick and bubbly.

Step 10

Remove from the heat and stir in the vanilla and butter. Pour the custard into a bowl, place a piece of plastic wrap on the top of the custard and set aside (or in the refrigerator) to cool.

Step 11

Pour the chocolate chips and corn syrup into a medium sized bowl. Bring the heavy cream to a boil, and pour the cream over the chocolate mixture. Allow to sit for a few minutes, and then whisk until smooth and creamy.

Step 12

Place the ganache in the refrigerator to cool before piping onto the cupcakes.

Step 13

Using OXO's filling tool, pipe a bit of the custard filling into the middle of each cupcake, being careful not to pipe too much so that the cupcake bursts! If you don't have the filling tool, you can slice each cupcake in half and add a hefty dollop of the cream.

Step 14

Clean out the decorating tool, fill with the ganche, and pipe a thick layer over the tops of your cupcakes. Or, just spread it on.

Step 15

Serve at room temperature, but store in the refrigerator for 2-3 days.