Decorate cake. Prepare two batches buttercream icing following recipe instructions. Prepare decorating bag with coupler and white icing. Using tip 2, pipe houses, icicles, dots and trees on sides of cake. Reserve remaining icing in bag.
Using spatula, ice top of cake. Use tip of spatula to create icing marks to make icing look like snow.
Switch tip on decorating bag to tip 10. Pipe snowdrifts coming over top edge of cake, then smooth with tip of spatula.
Garnish with rosemary sprigs, if desired.