Place cooled cake on a wire rack over a sheet pan.
Use a long serrated knife to cut cake in half horizontally.
Slide top of cake onto the back of a sheet pan.
Spoon frosting over cake.
Spread evenly.
Replace top layer of cake.
Use a 5-inch Christmas tree cookie cutter to cut out Christmas tree shapes.
Working with one cake at a time, spoon fondant around edges and down sides to cover.
Fill in top.
If you run out of fondant, scrape the fondant from the bottom of the pan back into the bowl, heat in the microwave for 15-20 seconds to loosen it back up, and strain through a fine mesh sieve. Repeat as many times as needed.
Allow to dry completely.
Color a small amount of remaining fondant with red food coloring, put in a piping bag, and pipe a zigzag on the trees.
Top with green coarse sugar.