Preheat your oven to 350°F (175°C) and line your cupcake pans with paper liners.

In a clean mixing bowl, beat the egg whites with a hand mixer until stiff peaks form, about 4-5 minutes. Set aside.

Cream together the unsalted butter and granulated sugar until light and fluffy, about 3-4 minutes.

In a medium bowl, whisk together the all-purpose flour, baking powder, and sea salt. Gradually add to the butter mixture alternately with champagne.

Gently fold in the diced fresh peaches and whipped egg whites until incorporated.

Fill cupcake liners about two-thirds full and bake for 18-22 minutes, or until a toothpick comes out clean.

Simmer the heavy cream in a saucepan over medium heat. Whisk together cornstarch and some warm cream, then add back to the pan.

Stir continuously until thickened, about 5-7 minutes. Remove from heat and whisk in eggs and champagne.

Once the cupcakes are cool, cut a cone out of the center of each and fill with pastry cream.

For the buttercream, cream the remaining butter and peach flavoring until fluffy. Gradually add powdered sugar and milk.

Frost each cupcake generously with peach buttercream and decorate with nonpareils or mint leaves.