Place the egg yolk in a medium bowl and set aside.

In a medium saucepan, add sugar, cornstarch, and champagne. Cook over medium heat until the mixture is very hot, but not boiling.

Ladle out some of the hot liquid, and whisking the yolk continuously, pour into the egg yolk.

Pour the egg yolk mixture into the saucepan and whisk constantly over medium heat until the mixture thickens, about 10 minutes.

Remove from heat and stir in butter and salt. Pour into a fresh bowl and strain if desired. Let cool, then chill in the fridge for 2 hours.

Preheat oven to 350°F (180°C) and line a muffin tin with cupcake liners.

Whisk flour, baking powder, and salt together in a medium bowl.

In a large bowl, mix sugar, oil, and vanilla extract. Add eggs and mix until combined.

Mix in a third of the dry ingredients, then half of the champagne. Alternate until combined.

Fold in the diced peaches and fill cupcake liners ¾ full. Bake for 14-16 minutes or until a toothpick comes out clean.

Mix butter and shortening, then add powdered sugar.

Add peach schnapps and thin with champagne to desired consistency.

Divide buttercream, colouring one peach and the other pinky-red.

Pipe lines of each colour onto plastic wrap, roll, and place into a piping bag.

Remove the center of the cupcake and fill with champagne pastry cream. Pipe frosting on top.

Store cupcakes in the refrigerator for up to 5 days.