Preheat the oven to 350°F.
Grease an 8" x 8" baking dish with butter or cooking spray.
Combine the dry ingredients into a mixing bowl. In a separate bowl, whisk all the wet ingredients together.
Slowly add the dry ingredients to the wet mixture, stirring as you go. Mix until just combined.
Fill the greased baking dish with your brownie mixture and spread out evenly. Use a spatula to level the top.
Bake for 22-30 minutes, depending on how fudgy you want them to be. Leave to cool completely.
Whisk butter, powdered sugar, vanilla extract and food coloring if you are using to make your buttercream. Place into your pipung bag and set aside.
Cut the brownies into triangles.
Pipe buttercream in a tree pattern and decorate to look like Christmas trees.