To pipe buttercream trees on cake:
Ice cake with a smooth layer of buttercream. With tree-shaped cookie cutters, mark an overlapping pattern lightly in buttercream, to serve as a guide for piping trees. Fill a pastry bag fitted with a large star tip (such as Ateco #826) with buttercream. Beginning at bottom of a tree, position pastry tip at a 45-degree angle just above surface of cake, then squeeze while pulling forward and then up, releasing pressure to make a pointed tip. Repeat process to form lowest level of branches. Continue piping, making sure to cover ends of previous row as you work toward top of tree.