Preheat an oven to 350°F. Line a standard 12-cup muffin pan with paper or foil liners.
In a bowl, whisk together the flour, baking powder and salt. In a large bowl, using an electric mixer, beat together the sugar and butter on medium-high speed until light and fluffy, 2 to 3 minutes. Reduce the speed to low and add the egg and egg white one at a time, beating well after each addition, then beat in the vanilla. Add the flour mixture in 3 additions, alternating with the champagne in 2 additions, beating until just combined; scrape down the sides of the bowl as needed. Increase the speed to medium-high and beat just until no traces of flour remain, about 30 seconds; do not overbeat.
Divide the batter evenly among the prepared muffin cups, filling each about three-fourths full. Bake until the cupcakes are lightly golden on top and a toothpick inserted into the center of a cupcake comes out clean, 18 to 20 minutes. Transfer the pan to a wire rack and let cool for 5 minutes. Then transfer the cupcakes to the rack and let cool completely, about 1 hour.
To make the cupcakes, combine the confectioners sugar and butter in your stand mixer and paddle attachment. Mix on medium-high until smooth and creamy. Stop the mixer and scrape down the sides with a spatula. Add in the champagne and vanilla extract. Mix on medium-high speed for 8 minutes until light and fluffy. Once the cupcakes are cool, use a pastry bag and large pastry tip frost the cupcakes with the buttercream. (The frosted cupcakes can be refrigerated in an airtight container for up to 3 days; bring to room temperature before finishing.) Garnish the cupcakes with nonpareils and colored sugars and serve. Makes 12 cupcakes.