Preheat oven to 350 degrees F (180 degrees C).
Spray two cake pans with non-stick cooking spray. Line with parchment paper cut in a circle to fit bottom of pan. Spray with cooking spray again. Pour prepared Devil's Food Cake Mix (1 package) into pans, dividing evenly.
Bake for 35 to 40 minutes or until a toothpick inserted into the center of the cake comes out clean. Let cool completely.
Once the cakes are cool, use a long bread knife to slice off the muffin top, so your cake is flat on top. Cover cakes with plastic wrap and refrigerate for a couple of hours.
Meanwhile, line a large cookie sheet with parchment paper. Place candy melts in a large plastic pastry bag. Microwave on high for 1 minute. Gently squeeze back to break up the Green Candy Melts (1 cup). Microwave for 30 more seconds. Make sure it's completely melted.
Cut a small corner off the pastry bag, then pipe your Christmas trees onto the parchment paper. They don't have to be perfect. Start with a thick line going up and down. Then go to the left and right, piping the branches of the tree. Add Sprinkles (to taste). Refrigerate for 30 minutes.
Remove cakes from pans. Place the first cake on cake plate, cut side up. Add Vanilla Frosting (1 jar) to top of the cake. Place the other cake on top, cut side down. Press gently. Fill in any gaps on the side/middle of the cake with frosting.
Continue frosting the entire cake. Smooth the sides with an offset icing spatula. Gently add chilled candy Christmas trees to the side of cake and on top, if desired. Serve immediately or store in refrigerator for up to 5 days.