1 (15.25-ounce) package white cake mix (such as Duncan Hines)
4 large eggs
½ cup sour cream
1 ¼ cups pink champagne, divided, plus more if needed
½ cup freeze-dried strawberries
1 cup (8 ounces) unsalted butter, softened
4 cups powdered sugar
2 teaspoons vanilla extract
¼ teaspoon kosher salt
Nonpareils, for decorating
Preheat oven to 350°F. Lightly grease 2 (8-inch) round cake pans with cooking spray, and line bottoms with parchment rounds. Lightly grease parchment with cooking spray.
Add cake mix to the bowl of a stand mixer fitted with a whisk attachment. Add eggs, sour cream, and 1 cup of the champagne; mix on low speed until combined, about 30 seconds. Increase speed to medium, and beat 2 minutes. Divide batter between prepared pans. Bake in preheated oven until edges begin to pull away from sides of pan and a toothpick inserted in center comes out clean, about 20 minutes. Remove from oven, and let cool in pans on a wire rack 10 minutes. Remove from pans to wire racks, and cool completely, about 1 hour.
Place freeze-dried strawberries in a food processor, and process until a fine powder, about 30 seconds. Remove 1 tablespoon of the powder, and set aside for garnish.
Add butter to the bowl of a stand mixer fitted with a paddle attachment. Beat on medium-high speed until light and fluffy, about 2 minutes. Add powdered sugar, vanilla, salt, remaining strawberry powder, and remaining ¼ cup champagne; mix on low until incorporated. Increase speed to medium, and beat until light and fluffy. Add additional champagne, 1 teaspoon at a time, if frosting is too thick.
Remove parchment rounds from cakes. Place one cake layer on cake plate, and spread with about 1 cup of the buttercream. Top with second layer, and press gently.
Spread a very thin layer of frosting over top and sides of cake (crumb coat). Chill 15 minutes. Remove from refrigerator, and spread remaining frosting evenly over top and sides of cake. Decorate with remaining 1 tablespoon strawberry powder and nonpareils.