Preheat oven according to brownie box instructions and spray 9-inch springform pan with oil.

Mix both brownie mixes together and pour into prepared springform pan.

Bake according to package directions until toothpick comes out clean (may take 5-10 minutes longer than box suggests for double batch).

Let brownies cool completely in pan (about 45 minutes to 1 hour), then release from springform and place on cutting board.

Cut brownie round into 8 equal triangular wedges.

In separate small bowls, mix white frosting with red gel coloring and green gel coloring to desired shades.

Fill three icing bags fitted with star tips with red, green, and white frosting.

For Christmas trees: pipe green frosting in rows from bottom to top creating tree shape, add white pearl sprinkles and gold stars. For Santa hats: pipe red triangle covering most of brownie, add white frosting trim along bottom edge and white pom-pom at tip.

Let decorated brownies sit 15-20 minutes before serving to allow frosting to set slightly.