Make the Cupcakes
Preheat oven to 350°F (175°C).
Line two cupcake pans with 16–18 paper liners.
Beat egg whites to stiff peaks and set aside.
In a separate bowl, cream butter and sugar until light and fluffy.
Sift together flour, baking powder, and salt.
Alternate adding the flour mixture and champagne to the butter mixture.
Fold in diced peaches.
Fold in one-third of the egg whites, then gently fold in the remaining egg whites.
Fill cupcake liners about ¾ full.
Bake for 18–22 minutes or until a toothpick inserted in the center comes out clean.
Cool 5 minutes in the pan, then transfer to a wire rack to cool completely.
Make the Champagne Pastry Cream
Whisk together ¼ cup heavy cream and cornstarch.
Add whole egg and egg yolks; whisk until smooth.
In a saucepan, combine remaining heavy cream, sugar, and champagne.
Bring to a boil and remove from heat.
Slowly whisk one-third of the hot mixture into the egg mixture.
Return remaining champagne mixture to a boil.
Gradually whisk in the tempered egg mixture until thickened.
Remove from heat and stir in butter and vanilla.
Cool to room temperature.
Cover surface with plastic wrap and refrigerate until needed.
Make the Peach Buttercream
Beat butter and confectioners’ sugar until combined.
Add milk or cream gradually until light, fluffy, and pipeable.
Add peach flavoring and mix well.
Tint frosting with yellow and orange gel food coloring until a peach color is achieved.
Fit a piping bag with a large star tip.
Paint two stripes of red gel coloring inside the piping bag for the peach-peel effect.
Fill bag with buttercream and set aside.
Assemble the Cupcakes
Cut a small hole in the center of each cupcake and reserve the cake pieces.
Fill each cavity with 1½–2 tablespoons of champagne pastry cream.
Trim reserved cake pieces and replace them as lids.
Pipe buttercream swirls on top.
Immediately sprinkle with white nonpareils.
Garnish with fresh mint leaves before serving, if desired.