Step 1

Preheat oven to 350 degrees F. Coat the bottoms of 2 (8-inch) cake pans with cooking spray.

Step 2

To make the cake, in a large bowl with an electric mixer on medium speed, beat cake mix, sour cream, water, oil, eggs and lemon zest until thoroughly combined. Pour batter evenly into cake pans. Bake 25 to 30 minutes or until a toothpick inserted in center comes out clean.  Let cool 10 minutes in the pans, then invert onto wire racks to cool completely.

Step 3

Meanwhile, to make the Frosting, in a large bowl with an electric mixer on medium speed, beat butter and vanilla until creamy. Gradually mix in confectioners’ sugar and milk until frosting is light and fluffy. If the frosting is a bit too stiff, you may need to add an additional tablespoon or two of milk.

Step 4

If desired, drizzle each cake layer with 2 tablespoons elderflower liqueur (see note). Place one cake, flat side up, on a cake platter. Spread a thin layer of frosting on top. Spoon lemon curd over frosting, making sure that you do not spread it all the way to the edge, or it will ooze out.

Step 5

Place second cake layer over first, (again, flat side up) and frost top and sides with remaining frosting. Decorate as desired. Chill at least 1 hour before serving.