Beat first 6 ingredients in a large bowl at medium speed with an electric mixer until blended. Stir in buttermilk.
Stir together one cup hot water and baking soda; stir into batter. Stir in vanilla. Pour into 2 greased and floured 9" round cake pans.
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.
To prepare Buttercream Frosting, beat butter at medium speed with an electric mixer until fluffy; gradually add powdered sugar, beating until light and fluffy. Add milk, beating until spreading consistency. Stir in vanilla.
Stir in orange food coloring into 1 1/2 cups frosting. Stir in yellow food coloring into 1 1/4 cups frosting. Makes 3 1/2 cups.
Pipe Buttercream 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.