Preheat the oven to 350 degrees.
Line a 9x13 baking dish with foil and spray with non-stick cooking spray.
Combine the brownie mix and ingredients, mixing well.
Pour the brownie batter into the prepared baking dish.
Bake for 18-22 minutes.
Remove from oven and let cool completely.
Combine the powdered sugar, softened butter and vanilla extract. Add in 1 tablespoon of milk at a time, as needed until the frosting as reached your desired consistency.
Attach the star tip to the piping bag and fill the bag with green frosting.
Use the 3 varying cookie cutters to gently cut out the brownie circles.
Gently squeeze the frosting out of the bag and onto the top of each brownie (I started with the biggest and stacked).
Top the last brownie with frosting to resemble to top of a Christmas tree.
Add a star on top and holiday sprinkles.
Repeat with the rest of the brownies.