Preheat the oven to 350°F, line the cupcake pan, and cream your sugar and butter for 3-4 minutes until light and fluffy.

Add vanilla extract, sour cream, and egg whites, scraping down the bowl's sides between additions.

Combine dry ingredients in a separate bowl and mix water and milk in a measuring cup. Gradually add dry ingredients and water-milk mixture to the batter.

Fill the cupcake liners halfway with batter and bake for 15-17 minutes. Allow your cupcakes to cool in the pan for 2-3 minutes before transferring to a cooling rack.

Beat egg yolks, then heat milk, cornstarch, and sugar until thickened in a saucepan. Simmer for 2 minutes.

Introduce some milk mixture to egg yolks, then return to the pan and boil for 2 more minutes.

Off the heat, add vanilla extract and butter to the pastry cream and cool to room temperature.

Combine chocolate chips and corn syrup in a bowl and pour hot cream over them. Let the mixture sit for 3-5 minutes, then whisk until smooth and refrigerate for 1-2 hours.

Cut out cupcake centers to fill them with pastry cream, topped with the now-cool chocolate ganache.

Refrigerate until serving. For optimal taste, serve your cupcakes close to room temperature and within 3 to 4 days.