• Preheat the oven to the temperature recommended on the box of brownies mix.Prepare a 9X9 baking pan (preferably a light metal pan) by lining it with parchment paper. Leave extra parchment paper over the edges so that you may easily remove the whole pan of brownies later.
• Whisk together the ingredients recommended on the box of brownies, and add the additional egg yolk. Mix just until combined, avoid over mixing.
• Bake the brownies on the middle rack in your oven according to package directions. Take care not to overbake.
• Remove brownies from oven and place on a wire rack to cool for 10 minutes. Then place in the refrigerator to cool completely.

Decorating

• While the brownies are cooling, place the frosting in a bowl, add food coloring and mix well. Make sure to scrape down the sides to make sure all the frosting is evenly colored.
• Remove cooled brownies from the pan and trim off just outer edges. There are 2 different ways to cut these depending on how large you want the trees to be.  If you want small trees, you can get 18.  Cut the brownies into 3 long strips.  Then cut the strips into triangles – this will make 3 rows with 6 triangles each. Cut 3 equal squares from each strip, then cut the squares in half on the diagonal. They will be small trees.  If you want large trees, you can get 8.  Cut the brownies in half down the middle.  You will be able to cut 4 equal size triangles from each half. It will only work if you trim a little off of each end, at an angle.
• Decorate with green icing and sprinkles. Stick a cut candy cane in the bottom of the triangle to be the stem of your Christmas tree.