Prepare and bake brownie mix according to directions for 13 x 9-inch pan and cake variation using an additional egg. Let cool completely. Cut seven circles from baked brownie in the following sizes: 6 inches, 4-3/4 inches, 3-1/2 inches, 2-3/4 inches, 2 inches, 1-3/4 inches, and 1 inch. Stack circles to make tree. Push a bamboo skewer through the center of the tree to secure the layers, trimming if it sticks out of the top.
To decorate you'll need 1 can whipped vanilla frosting and green, yellow, blue, and red icing colors. Place 1/2 can (6 ounces) of vanilla frosting into a bowl and tint it bright green using green and yellow icing colors. Divide remaining icing between two bowls. Tint one portion blue and the other red. Place green icing in a pastry bag fitted with a medium star tip, blue icing in a pastry bag fitted with a #4 writing tip, and red icing in a pastry bag fitted with a #6 writing tip. Once tree is assembled, pipe a "garland" of green stars around base of each brownie layer. Randomly pipe red and blue dots onto brownie layers. Make a star out of a soft red candy.