Step 1

For the champagne reduction, bring champagne to a simmer in a heavy saucepan over medium low heat. Continue to simmer, stirring occasionally, until champagne is reduced to 1 cup (about 10 minutes). Pour into a glass measuring cup and let cool to room temperature.

Step 2

For the curd, whisk together citrus zest and juice, sugar, and eggs in a heavy saucepan. Cook over moderately low heat, whisking constantly, until slightly thickened. Add butter a few pieces at a time, whisking constantly until melted, before adding more butter. Continue until curd is smooth and thickened, and coats the back of a spoon (about 5 minutes total). Strain curd through a fine mesh sieve into a bowl. Press a piece of plastic directly on the surface, and chill until cold – at least one hour.

Step 3

For the cake, preheat oven to 350°F and butter and flour three 8-inch round cake pans. Whisk together flour, baking powder, baking soda, and salt in a medium bowl.

Step 4

Beat butter and sugar in a large mixer bowl at medium speed until light and fluffy. Beat in citrus zest and vanilla until incorporated. Add eggs one at a time and beat until combined. Reduce speed to low and add flour in 3 additions, alternating with buttermilk and champagne reduction (begin and end with flour mixture).

Step 5

Divide batter evenly between cake pans and smooth with an offset spatula. Tap pans sharply to reduce air bubbles.

Step 6

Bake for 24 to 26 minutes, until the cakes are golden, a toothpick inserted in the center comes out clean, and the edges are pulling slightly away from the pan. Let cakes cool in pans for 15 minutes before inverting onto wire racks to cool completely.

Step 7

For the frosting, beat butter and salt in a large mixer bowl on medium speed until smooth. Mix in 2 cups of powdered sugar on low speed until incorporated. Mix in champagne reduction and vanilla until incorporated. With mixer on low speed, add remaining powdered sugar 1 cup at a time and beat well, scraping down the bowl as needed. Increase speed to medium and beat frosting for 1 minute, until light and airy.

Step 8

To assemble the cake, place one layer on a rotating cake stand. Use an offset spatula to swirl frosting over the cake layer. Transfer 1 cup of frosting to a large piping bag and pipe a ring of frosting around the outer edge of the cake layer. Use a spoon to spread about 3 tablespoons of citrus curd in the center of the cake layer. Add second cake layer and repeat with frosting and citrus curd. Add top cake layer and chill for 20 minutes.

Step 9

Use offset spatula to apply a layer of frosting to the top and sides of the cake while you rotate the cake stand, then use a bench scraper to gently smooth the edges of the cake and remove any excess frosting.