Sandwich the cakes with the butter icing to make the snowman's body and stand upright on a cake board or plate. Spread jam thinly all over the outside of the cakes to stick the crumbs in place. Press the muffin on to the body so that its domed part forms the snowman's face. Spread the muffin with the remaining jam.
Cover the cakes completely with royal icing, spreading with a round-bladed knife and pulling it into peaks with the back of the knife.
Knead the ready-to-roll icing on a surface lightly dusted with icing sugar until slightly softened, keeping the colours separate. Shape an orange carrot nose and black pebble eyes, a black mouth and black buttons. Press on to the snowman. Shape half the red icing into a round and press on to the snowman's head for a hat. Use a strip of yellow icing as a hatband.
Shape the rest of the yellow icing into a rope 18 cm (7 inches) long. Repeat with the remaining orange and red icing. Twist together, roll out to flatten and trim to a 36 x 2.5 cm (14 x 1 inch) scarf. Make small cuts in either end for a fringe, then wrap around the snowman. Re-knead and roll out the trimmings, fringe one side, then roll up and add to the top of the hat for a bobble. Dust with sifted icing sugar for freshly fallen snow.