Tint 1½ cups buttercream with red gel until bright barn red. Reserve ½ cup plain for trim.
Stack cakes with a thin layer of buttercream between; chill 15 minutes to firm.
Crumb-coat entire cake with plain buttercream; chill 10 minutes.
Spread red buttercream evenly for barn walls. Use an offset spatula for smooth sides.
Pipe roof with chocolate frosting in rows or use a spatula to make a shingled look.
Toast coconut 3–5 minutes in a 350°F oven for “hay,” if using.
Add licorice or piping gel for doors and windows. Place animal toppers on and around the barn.
Scatter green sugar or sprinkles for a yard and press coconut near the base.