The perfect Easter dessert! For the decorating technique, see our step-by-step photos.
Preheat oven to 350 degrees. Butter both halves of an 8-cup lamb-shaped cake mold. Dust with flour; tap out excess.
Sift flour, sugar, baking soda, baking powder, and salt together into the bowl of an electric mixer. Add egg and yolk, 3/4 cup warm water, the buttermilk, oil, and vanilla. Beat on low speed until smooth.
Pour batter into halves of molds. Bake until a tester comes out clean, 35 to 40 minutes. Let cool in molds on wire racks 30 minutes. Unmold; let cool completely.
Trim flat side of each. Trim excess from edges. Cover flat side of 1 cake with jam. Turn halves upright; stick together.
Spread a thin layer of pale-yellow buttercream over cake, sealing edges. Refrigerate 30 minutes. Spread with a second layer, smoothing around head.
Cover with chocolate curls (exclude head). Put brown buttercream in a pastry bag fitted with a small plain round tip; pipe eyes and mouth. Refrigerate 30 minutes or up to 2 days.