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.
3/4 cup sugar, 1/4 cup (3/4 stick/85g) unsalted butter
Reduce the speed to low, add the egg and egg white one at a time, beating well after each addition, then beat in the vanilla.
1 whole egg, 1 egg white, 1 tsp vanilla extract
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 and be careful not to over beat.
1/2 cup (125ml) champagne
Divide the batter evenly among the prepared muffin cups, filling each about three-fourths full (you should get about 11-12 cupcakes).
Bake for 18-20 minutes or until a toothpick inserted into the center comes out clean.
While the cupcakes are baking; in a small saucepan, bring the champagne to a boil over medium-high heat. Reduce to medium and allow to simmer until it reduces down to about 1/4 cup (about 8-10 minutes). Reserve 2 tablespoons of the reduction for the frosting, and use the rest to brush on top of the warm cupcakes. I brushed each cupcake twice with the reduction and discarded the leftover. Let cupcakes cool completely before frosting.
1 cup (250ml) champagne
In large bowl, using an electric mixer, beat the butter, powdered sugar, vanilla and 2 tablespoon of the champagne reduction until smooth. Frost as desired and sprinkle with sparking sugar.