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 it thickens and coats the back of a wooden spoon, about 10 minutes.

Remove from heat and stir in butter and salt. Pour the pastry cream into a bowl. Let cool at room temperature, then chill it in the fridge for at least 2 hours.

Preheat oven to 350°F (180°C). Line a 12 cup 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 together. Add eggs and mix until combined.

Mix in a third of the dry ingredients, then half of the champagne. Add in another third of the dry ingredients, then the rest of the champagne. Mix together.

Pour in remaining flour and mix until the flour is just incorporated. Add the diced peaches and mix until well distributed.

Fill the cupcake liners ¾ full. Bake for 14-16 minutes, or until a toothpick comes out clean.

Add butter and shortening to a large bowl and mix until well combined. Add powdered sugar and mix until incorporated.

Add peach schnapps and mix together. Add 1-3 tablespoons of champagne to thin the buttercream until pipeable.

Divide the buttercream, adding food coloring to one half for a peach color and red food coloring to the other half for a pinkish-red color.

Pipe both colors side by side onto plastic wrap, roll up, and cut the end to place in a piping bag.

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

Store cupcakes in the refrigerator for up to 5 days.