In a small saucepan, bring 1 cup of champagne to a boil over medium-high heat.
Once boiling, reduce to medium heat and allow to simmer until reduced down to a little less that ½ cup, about 15 minutes.
Set aside to cool completely. You want it to be room temperature or cold before adding it to the buttercream. You can let it cool in the fridge or make it up to 3 days in advance.
Cupcakes
Preheat the oven to 350°F and line a 12 cup cupcake pan with cupcake liners.
Whisk the cake flour, baking powder, and salt together in a large bowl.
In a separate bowl, whisk the melted butter, sugar, eggs, vanilla, and champagne reduction together until combined.
Stir in about half of the flour mixture. Then stir in the milk and the rest of the flour mixture and mix until fully combined. The batter will be thin.
Pour about ¼ cup of batter into each cupcake liner (they should be about halfway full). Bake at 350°F for 18-22 minutes or until a cake tester comes out clean.
Remove from pan and let cool on wire rack. Let cupcakes cool completely before frosting.
Champagne Buttercream
In a stand mixer fitted with the paddle attachment, cream together the softened butter, powdered sugar, gold luster powder, and vanilla until smooth and creamy.
Add in the champagne reduction 1 tablespoon at a time until fully combined. Beat the buttercream for 3-4 minutes on high speed until light and fluffy.
Frost cooled cupcakes and enjoy!
Notes
These cupcakes will last in the fridge for up to 3 days. However, they taste best when served at room temperature! I like to let them sit at room temperature for about an hour before serving.