Bake and prepare brownies in 13x9 baking pan. Let cool.
While the Brownies bake, prepare frosting. Combine butter and vanilla until smooth. Gradually add in powdered sugar and mix until piping consistency.
Once desired consistency is reached, add in 3 drops of green food coloring, mix in completely. Put in pastry bag and cut off small corner.
Cut cool brownies into 3 horizontal rows. Then cut diagonally one way 4 times and then another 4 time the opposite way to create a triangle shape.
Lay out cut brownies into serving platter, insert pretzel sticks into bottom to look like a stump. Then drizzle with green icing and decorate with Christmas sprinkles to give appearance of a Christmas tree.