Preheat your oven to 350°F (175°C). Line a 9-inch square baking pan with parchment paper, leaving an overhang on two sides. Lightly grease the parchment paper.
In a large bowl, prepare the brownie batter according to the package directions, using the water, oil, and eggs required.
Pour the batter into the prepared pan and use a spatula to spread it into an even layer.
Bake for 28 to 32 minutes, or until a toothpick inserted 2 inches from the side of the pan comes out with moist crumbs attached.
Let the brownies cool completely in the pan on a wire rack, which will take about 1 hour. For cleaner cuts, place the cooled brownies in the freezer for 15 minutes.
Use the parchment overhang to lift the brownie slab out of the pan. Place it on a cutting board and cut it into three long, even rows. Then, cut each row into five triangles.
Gently break the curved part off each miniature candy cane. Insert the straight piece into the base of each brownie triangle to create a tree trunk.
Place the vanilla frosting in a small bowl and add a few drops of green food coloring. Stir until the color is uniform.
Transfer the green frosting to a small zip-top bag and snip off a tiny corner. Pipe a zigzag line across each brownie triangle to look like a garland.
Immediately add holiday sprinkles on top of the frosting before it sets. Let the brownies sit for a few minutes for the frosting to firm up before serving.