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 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 fresh medium bowl. Let cool at room temperature and then chill 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 the remaining flour and mix it in a bit. Add the diced peaches and mix the batter until the flour is just incorporated, and the peaches are well distributed.

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

Add butter and vegetable 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 it's pipeable.

Remove half the buttercream to a separate bowl. Add a couple of drops of yellow and orange food colouring to one bowl to make a peach colour.

Add red food colouring to the other bowl of buttercream to make a pinky-red colour.

Pipe a long line of peach buttercream onto a piece of plastic wrap. Pipe a long line of pinky-red buttercream right alongside it. Roll the buttercream up with the plastic wrap. Cut the end and place it into a piping bag.

Use a knife, melon baller, or cupcake corer to remove the center of the cupcake. Fill the centers with the champagne pastry cream. Pipe the buttercream frosting onto the tops of the cupcakes.

Store cupcakes in the refrigerator for up to 5 days.