Remove the frozen layers from the freezer and unwrap. If they are uneven, use a sharp, serrated knife to level them. Place the bottom layer on a 10-inch cake board and set on a turn-table.
Separate some of the frosting into a separate bowl for the crumb coat. Taking from this "dirty" bowl, place a dollop of frosting on the cake and smooth it around using a 6-inch offset spatula.
Place the next layer on top and repeat the previous two steps with the remaining cake.
Using the offset spatula and only taking from the dirty bowl, frost a thin layer of buttercream all around the cake. Get it as smooth as possible. If the frosting gets too cold from the frozen cake, dip your offset spatula in a bowl of hot water, wipe dry, and continue frosting. Hot tools can be incredibly helpful with a stiff buttercream.
When the crumb coat is finished, place the cake in the refrigerator to set up for about 10 minutes.
When the crumb coat is set, continue frosting the cake. Fit a piping bag with Wilton Cake Icer Decorating Tip 789. Using the flat edge, pipe ribbons of frosting all around the cake, starting at the bottom and working your way up.
Hold an icing smoother at a 45° angle to the cake. Lightly hold in place on the frosting as you rotate the turn-table to smooth the frosting. Again, it may be helpful to heat up the smoother depending on how stiff the frosting is. Continue smoothing the cake until you are satisfied. If needed, fill in gaps or air pockets with the offset spatula.
Place a dollop of frosting on the top of the cake and smooth with the offset spatula.
To achieve a perfectly smooth cake, dampen a Viva paper towel (this is the only brand that works) and gently place it on the cake. Use a fondant smoother to "iron" out the frosting.
Take the remaining frosting and add green food coloring until it is as dark as you would like.
Put the green frosting in a piping bag fitted with Wilton Tip 18.
Take a waffle cone and pipe stars of frosting all over it, staring from the bottom and working up. Place the cone on top of the cake. Repeat with the other two cones.
When all the trees are set, put some powdered sugar in a fine mesh strainer and sift a dusting of powdered sugar over the cake.
Transfer to a cake stand before serving. The cake should be stored in the refrigerator, but brought to room temperature before serving. Enjoy!