1. Preheat oven to 325 degrees. Butter two 8-inch round cake pans. Line with parchment. Butter paper, and flour pans. Beat together butter and sugars with a mixer until light and fluffy, about 3 minutes. Beat in eggs, 1 at a time, beating well after each addition. Beat in vanilla. Whisk together flour and salt. In another bowl, combine sour cream and baking soda. Gradually add flour mixture to butter mixture in 3 additions, alternating with sour cream mixture and starting and ending with flour mixture.

2. Divide batter between pans. Bake until golden and a toothpick inserted into centers comes out clean, about 50 minutes. Let cakes cool in pans, about 30 minutes. Invert cakes, remove pans and parchment, and let cool completely on a wire rack.

3. Trim tops of cakes with a serrated knife to make level, then cut each cake in half horizontally. Reserve 1 bottom layer for another use. Place remaining bottom layer on a cake stand, and spread evenly with 1 1/4 cups frosting. Repeat with a second layer and another 1 1/4 cups frosting. Place third layer on top. Spread entire cake top and sides with 1 1/4 cups frosting. Refrigerate cake until firm, about 30 minutes. Spread with 1 1/2 cups frosting, smoothing top and sides. Serve immediately with marzipan animals, or refrigerate up to 2 days; if refrigerated, let cake come to room temperature before serving.