Beat first 6 ingredients in a large bowl at medium speed with an electric mixer until blended. Stir in buttermilk.
Stir together 1 cup hot water and baking soda; stir into batter. Stir in vanilla. Pour into 2 greased and floured 9-inch round cakepans.
Bake at 350° for 30 to 40 minutes or until a wooden pick inserted in center comes out clean. Cool in pans on wire racks 10 minutes; remove from pans, and cool completely on wire racks.
Freeze layers 30 minutes. Cut each layer into 8 wedges.
Pipe frosting on top and sides of cake wedges to resemble candy corn. Using a medium star tip, pipe white frosting on the small end of each cake, yellow frosting on center, and orange on wide end.