For the brownies: Preheat the oven to 350 degrees F. Butter the bottom and sides of two 8-inch round cake pans. Cut two 8-inch circles out of parchment and line the bottoms of the cake pans with the parchment.
Stir together the butter, granulated sugar, vanilla and eggs in a medium bowl. In another bowl, whisk together the flour, cocoa powder, salt and baking powder. Add the dry ingredients to the wet ingredients and stir until just combined. Stir in the chocolate chips and divide the batter between the prepared pans, smoothing the tops. Bake until the brownies begin to gain a crust around the edges, pull away from the edge and a toothpick inserted in the center comes out clean, 35 to 40 minutes.
Let the brownies cool fully in the pans on cooling racks, about 1 hour. Cut each circle into 8 equal triangles with a knife. Use a small offset spatula or pie server to lift each triangle out of the pan.
For the American buttercream frosting: Combine the confectioners' sugar, butter and salt in the bowl of a stand mixer fitted with a paddle attachment (or in a large bowl if using a handheld electric mixer). Mix on low speed until mostly incorporated. Add the vanilla, increase the speed to medium-high and mix until smooth. Loosen the consistency with milk as desired. Add 24 drops green food coloring and 1 drop red food coloring and mix on medium speed for 1 minute. Set aside.
To assemble: Using the tip of a paring knife or a skewer, poke a small hole along the bottom edge of each brownie. Insert a candy cane "trunk" into each hole.
Fill the piping bag with the frosting and cut 1/4 inch off the tip. Using a zig-zag motion, pipe the frosting from the base to the tip of each tree. Decorate the trees with the sanding sugars and candy-covered chocolate, as desired.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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