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. You want to leave extra parchment paper over the edges so that you may easily remove the whole pan of brownies later.
Use a whisk to stir together the ingredients recommended on the box of brownies and 1 egg yolk. (If this is too challenging, then use a spatula). We don’t want to over-mix, so mix just until the flour disappears.
Bake the brownies on the middle rack in your oven.
Check the brownies a few minutes before the minimum time recommended on the box. You do not want to over bake them. Insert a toothpick to test doneness. There should be some streaks of batter on the toothpick. If they are not at this point yet, give them just a few more minutes at a time.
Leave the brownies in the pan and place it on a cooling rack. After a few minutes, move the pan into the fridge.
While the brownies are cooling, mix together the white frosting and green food coloring.
Remove the brownies from the pan. Trim off outer edges.
Do not cut the brownies unless they are completely cooled - if they are not - return them to the fridge and wait until they are completely cooled.
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 rinto 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 is like a zig-zag pattern.) 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.