Bake and cool cake. Place cake on turntable for ease of icing. Fill a large pastry bag (at least 14") fitted with large icer tip with buttercream icing. Pipe icing directly onto cake, pressing the tip gently against the cake sides. Pipe icing on the top of the cake, completely covering the cake. Smooth the sides, then the top with a long metal spatula. When icing forms a crust, smooth out with a pastry roller.