Preheat the oven to the temperature recommended on the box of brownies mix.
Prepare a 9x9-inch baking pan by lining it with parchment paper. Leave extra parchment paper hanging over the edges so they can be easily removed from the pan later.
In a large bowl, whisk (or use a fork to mix) together the ingredients recommended on the box of brownies and 1 additional egg yolk. You don’t want to over-mix, so mix just until the flour disappears.
Transfer the batter to the prepared baking pan and back according to box directions. Check the brownies a few minutes before the minimum time recommended on the box. You do not want to overbake them.
Let the brownies cool in the pan on a wire rack.
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 - place them in the refrigerator and wait until they are completely cooled.
There are two different ways to cut these depending on how large you want the trees to look. You can get 18 small trees by cutting 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.
You can make 8 large trees by cutting the brownies in half down the middle. You will be able to cut 4 equal size triangles from each half. It will look like a zig-zag pattern and will work best 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.