Preheat the oven to 350°F (177°C). Line a 12-count muffin pan with cupcake liners. Line a second pan with 3 liners—this recipe makes about 15 cupcakes. Set aside.

Whisk the cake flour, baking powder, baking soda, and salt together. Set aside.

Using a handheld or stand mixer fitted with a paddle attachment, beat the butter on high speed until smooth and creamy—about 1 minute. Add the sugar and beat on high speed for 2 minutes until creamed together. Scrape down the sides and up the bottom of the bowl with a rubber spatula as needed. Add the egg whites and the vanilla. Beat on medium-high speed until combined, then beat in the orange zest and sour cream. Scrape down the sides and up the bottom of the bowl as needed. With the mixer on low speed, add the dry ingredients until just incorporated. With the mixer still running on low, slowly pour the the orange juice and champagne in until combined. Do not overmix. You may need to whisk it all by hand to make sure there are no lumps at the bottom of the bowl. The batter will be slightly thick.

Pour/spoon the batter into the liners—fill only 2/3 full to avoid spilling over the sides. Bake for 18-21 minutes, or until a toothpick inserted in the center comes out clean. For around 30 mini cupcakes, bake for about 11-13 minutes, same oven temperature. Allow the cupcakes to cool completely before frosting.

In a small saucepan, bring the champagne to a boil over medium-high heat. Once boiling, reduce to medium heat and allow to simmer until reduced down to 1/4 cup, about 7-10 minutes. Set aside to cool completely. You want it room temperature or cold.

In a large bowl using a handheld or stand mixer fitted with a whisk attachment, beat the butter on high speed until completely smooth and creamy, about 2-3 minutes. Add the confectioners’ sugar 1 cup at a time, beating on low at first then increasing to high speed. Once incorporated, add the next cup. Once creamy and combined, beat in 3 Tablespoons of the reduced champagne and the vanilla extract. Taste. Add the remaining reduced champagne if needed, and a pinch of salt if you’d like.

Frost cooled cupcakes. Garnish with orange slices and gold sprinkles, if desired.