Make the Cupcakes
Preheat oven to 350°F (175°C) and line a 12-cup muffin pan with cupcake liners.
Beat the egg whites until stiff, glossy peaks form. Set aside.
In a large mixing bowl, cream together the butter and sugar until light and fluffy.
In a separate bowl, whisk together flour, baking powder, and salt.
Alternately add the dry ingredients and champagne to the butter mixture, beginning and ending with the dry ingredients.
Gently fold in the whipped egg whites and diced peaches.
Fill cupcake liners about two-thirds full.
Bake for 18–22 minutes, rotating the pan halfway through baking.
Cool completely on a wire rack.
Prepare the Pastry Cream Filling
In a bowl, whisk together ¼ cup heavy cream, cornstarch, egg, and egg yolks.
In a saucepan, combine remaining cream, sugar, and champagne. Heat until steaming but not boiling.
Slowly whisk the hot mixture into the egg mixture.
Return to the saucepan and cook over medium heat, stirring constantly until thickened.
Remove from heat and stir in butter and vanilla extract.
Cover and refrigerate until completely chilled.
Fill the Cupcakes
Core the center of each cooled cupcake.
Fill each cavity with chilled pastry cream.
Replace the cupcake tops.
Make the Peach Buttercream
Beat butter until creamy.
Gradually add confectioners’ sugar and peach flavoring.
Add milk or cream as needed until a smooth piping consistency is reached.
Decorate
Paint streaks of red gel food coloring inside a piping bag.
Fill the bag with buttercream and pipe decorative swirls onto each cupcake.
Sprinkle with white nonpareils to resemble champagne bubbles.
Garnish with fresh mint leaves if desired.
Refrigerate until ready to serve.