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 thickened, about 10 minutes.

Remove from heat and stir in butter and salt. Pour through a fine mesh strainer if desired. Let cool and then chill 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. Add eggs and mix until combined.

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

Pour in the remaining flour and mix until just incorporated. Fold in the diced peaches.

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

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

Add peach schnapps and mix together. Add champagne to thin the buttercream.

Divide the buttercream and color one half peach and the other pinky-red.

Pipe a long line of each color onto plastic wrap, roll it up, cut the end and place it in a piping bag.

Remove the center of each cupcake. Fill with champagne pastry cream and pipe the buttercream frosting on top.