Preheat oven to 350 degrees F (160 degrees C). Line muffin tin with cupcake wrappers.

Beat softened butter in a stand mixer (or with hand mixer) until smooth and light yellow, about 2 minutes. Add sugar and beat, scraping the sides of the bowl often, until fully incorporated, about 2 minutes. Add the egg whites, vanilla and sour cream and beat again, until fully incorporated.

Mix dry ingredients together.

With the mixer on low to medium speed, alternate adding the dry ingredients and the milk until fully incorporated, taking care not to overmix.

Fill each cupcake 2/3 full and bake for 18-24 minutes, until toothpick comes out clean. Cool completely before frosting.

Add softened butter to mixing bowl and beat until smooth. On low speed, add the powdered sugar and mix well. Add the Champagne one tablespoon at a time. Mix until smooth. Add the frosting to a piping bag and decorate the completely cooled cupcakes. If it's warm in your kitchen, you may need to refrigerate the frosting for 10-15 minutes before using.

Preheat oven to 200 degrees F (95 degrees C). Cover a baking sheet with parchment paper.

Add egg whites to electric mixer bowl with whisk attachment. Whisk until frothy. Add cream of tartar and whisk on medium high until soft peaks form.

Add sugar slowly until fully incorporated and stiff peaks form. Fold in the powdered sugar, being careful not to deflate the air in the mixture. Add the mixture to a piping bag and pipe shapes onto the parchment paper. Bake for 2 hours or until totally dry. Turn off oven and let sit until cool.