Cupcakes:

Preheat the oven to 350F. Line cupcake pans with 18 paper liners. Set aside.
In the bowl of an electric mixer, beat the egg whites to stiff peaks; set aside.
In another bowl of an electric mixer, beat together the butter and sugar until light and fluffy. In yet another bowl, sift together the flour, baking powder, and salt. Whisk to combine.
Add the flour mixture to the creamed butter mixture alternately with the champagne. Stir in the chopped peaches.
Fold 1/3 of the egg whites into the batter until incorporated. Then, fold in the remaining egg whites.
Spoon the batter into the prepared cups. Bake for 18-22 minutes, or until a toothpick tester inserted near the center comes out clean. Cool for 5 minutes in the pan, then transfer the cupcakes to a wire rack to cool completely.
Champagne pastry cream:

In a medium bowl, whisk together 1/4 cup of the heavy cream and the cornstarch. Whisk in the whole egg and egg yolks. Set aside.
Combine the remaining heavy cream, sugar, and champagne in a saucepan; bring to a boil then remove from the heat. Pour 1/3 of the hot champagne mixture into the egg mixture, whisking constantly so the eggs do not cook. Return the remaining champagne/heavy cream mixture to a boil. Pour in the hot egg mixture in a stream, whisking constantly until the mixture thickens. Remove from the heat and stir in the butter and vanilla.
Let cool to room temperature. Cover the surface of the pastry cream with plastic wrap so it doesn’t form a skin. Store in the refrigerator until ready to use.
Peach buttercream:

In the bowl of an electric mixer fitted with the whisk attachment, beat together the butter and confectioners’ sugar. Add milk or cream a little at a time until the mixture is light, fluffy, and of piping consistency. Add the peach flavor and beat again until incorporated. Add food color a little at a time until desired hue is achieved.
Fit a disposable piping bag with a large French pastry tip. Squeeze a little of the red food color into a condiment cup. Dip a kitchen-dedicated soft bristle brush into the red food color and paint a thick line inside the decorator tip and all the way up the length of the bag. Repeat this step for the opposite side of the bag.
Assembly:
Core the cupcakes by cutting out a small divot in the centers with a paring knife; save the pieces. Transfer the pastry cream to a disposable piping bag and fill each cupcake with about 1 1/2 to 2 tablespoons of the pastry cream. Trim the cut-out cake pieces flat to create “lids” and place them onto the filled divots.
Pipe a swirl of frosting onto a cupcake and immediately sprinkle a pinch of the white nonpareils on top. Repeat this process with each cupcake until all of the cupcakes are frosted and sprinkled. Just before serving, add a fresh mint leaf to each cupcake.