Preheat the oven to 350 degrees Fahrenheit. Coat a 9x13-inch baking pan with non-stick spray to prepare for the brownie mix.
Prepare the brownie batter according to the box directions.
Pour the mixture into the pan and bake according to the time recommendation on the box.
Place the pan of brownies on a wire rack and allow the brownies to cool completely. If you don't have one, you could set the brownie pan on a trivet so that air can circulate around the pan to help them cool faster.
Once the brownies have cooled, cut your brownie pieces into the shape of triangles by cutting the entire pan of brownies in half lengthwise. Then, divide the halves into triangles. Cut the rounded corners off and discard (AKA eat right away!).
Remove the triangle brownies from the pan and place them on parchment paper to decorate!
Begin preparing the frosting.
Use a mixer to beat the sticks of butter until smooth and creamy.
Add powdered sugar and beat to combine the sugar and butter until thoroughly mixed and fluffy.
Add pure vanilla extract and 1-2 tbsp of whole milk. Mix to combine.
Add the last tbsp of milk, if necessary, to create your desired consistency.
Add a generous amount of green food coloring and mix completely. Add more until you reach your desired shade of green. If the frosting becomes too thin, add 2 tbsp of powdered sugar at a time to thicken it. Do this again until the frosting thickens to your desired consistency.
To decorate, add a squiggle of green frosting to your tree brownies, and top them with Christmas sprinkles of your choice!