Bake brownies according to package directions in a square baking pan.
Allow to cool completely, then carefully remove brownies from pan and place on a large cutting board.
Cut the brownie in half, then cut triangles out of each side. Discard the four edge pieces.
Prepare your icing by mixing green food coloring into the premade vanilla icing until the desired color is reached.
Place the icing in a piping bag or zip-top storage bag and pipe icing onto each brownie.
Top with green decorating sugar and Christmas light sprinkles. Add a star sprinkle to the top of each tree.
Notes
The great thing about these brownies is that you can use whatever you’d like to decorate them. For this version, we used the sprinkles and decorating sugar but you can use any cute things you can find! Any holiday sprinkles work well and I love using things like M&Ms, chocolate chips, and even peppermint chips!