Preheat oven to 350°F. Spray two (6-inch round) 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 cake batter into pans, dividing evenly between the two pans. Bake for 35-40 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 (If you're in a hurry, you can skip this step, but it's easier to frost when it's chilled).
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 candy melts. 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. Make some trees tall, some short and some in between. Add sprinkles. Place pan in refrigerator for 30 minutes or until set.
Remove cakes from pans. Place first cake on cake plate, cut side up. Add frosting to top of cake. Place the other cake on top, cut side down. Press gently. Fill in any gaps on the side/middle of 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 one day. (Note: the colored candy melts don't do well in the refrigerator for much more than one day. They're fine to eat for up to 5 days but might change color a bit after more than one day.)