Bake a double batch of brownies according to the recipe and let them cool completely in the pan. It helps to line the pan with parchment paper so you can lift the brownies right out of the pan when it's time to cut them.

Cut the brownies into equal triangles. Trim off the edges if they're uneven.

Combine all the frosting ingredients in a bowl and stir until smooth. Divide the frosting between two bowls: one portion for the reindeer and Christmas tree, and one for the snowman. The frosting for the reindeer and tree should be stiffer, so add more powdered sugar to that portion than the one for the snowman, which needs to be thinner.

Transfer the frosting to piping bags fitted with a round tip and decorate the brownies.

To make the reindeer, pipe a little frosting onto the small pretzel twists so they stick to the cookie. Attach the red M&M for the nose the same way. Pipe two round eyes and let the frosting set completely before adding pupils, either with a dot of chocolate sauce or a food-safe marker.

To make the Christmas tree, pipe a garland of frosting and sprinkle on sanding sugar right away, before the frosting sets. Glue a star onto the cookie with a dab of frosting. Stick a dark brown gummy candy into the bottom of the cookie for the trunk.

To make the snowman, use the thinner frosting. Pipe an outline first around the area you want white, then fill it in with frosting. Gently shake the cookie from side to side to smooth the frosting into an even layer. Place a dark brown gummy candy along the edge for the hat brim and an orange gummy candy for the nose before the frosting sets. Once the frosting has set, draw the eyes with chocolate sauce or a food-safe marker.