Prepare brownie mix according to the instructions on the box.
Pour the brownie mix into the baking pan lined with parchment paper that is called for on your box (usually either 8x8-inch square baking pan or 9x13-inch rectangular baking pan).
Bake the brownies. Be sure not to over-bake.
Let the brownies cool in the pan. Then, lift the brownies out of the pan by the parchment paper and lay the brownies on a cutting board.
Trim edges if your pan has rounded sides to make them more square.
Cut the brownies in half lengthwise. Then, cut each half into a series of triangles. You may have some scraps that you will not need.
Frost half of the brownies with green icing to create Christmas tree brownies. Top with a yellow M&Ms at the tip of the triangle for the tree topper and add colorful sprinkles as ornaments all over the green icing. Stick a pretzel stick near the wider base of the triangle as the Christmas tree stump.
For the reindeer brownies, add two candy eyes near the wider side of the triangle. Then, add a red M&M near the tip of the triangle for the reindeer’s nose. Use icing to glue the candy eyes and red M&M to the brownies. Use two mini pretzel twists above the eyes for the reindeer antlers.
Serve and enjoy!