Prepare your brownie batter according to the package directions.
Line an 8"x8" or 9"x9" baking pan/pyrex with parchment paper/foil after greasing the bottom and sides of the dish with cooking spray or other.
Bake brownies according to the directions and allow to cool afterward for a minimum of 2 hours. I suggest lifting the brownies (with the parchment paper) from the pan after 30 minutes and allowing them to cool the remaining 1.5 hours on a cooling rack.
Gently remove the parchment paper or aluminum foil. Cut your brownie square into 3 equal rows. Then using a sharp knife, carefully cut each row in alternating diagonals to make triangles. Don't rush this step, brownies are delicate.
In a small bowl mix in your food coloring gel or drops into half the whipped buttercream frosting. Add food coloring until the desired color is obtained. My leaf green Wilton soft gel paste did not require much.
Unwrap your small candy canes and using your hands or a sharp knife, break off the candy cane hook, leaving just the straight portion.
Insert the sharpest part into the base of your brownie Christmas tree.
Transfer your buttercream into a piping bag, piping gun, or a ziplock baggy.
Start at the top of your brownie tree and work your way down squeezing your buttercream icing in a zigzag fashion.
Add your multi-colored sprinkles onto your buttercream in a similar pattern.
Add some sugar pearls to the top of each brownie tree.