Line a baking tin with baking parchment. Heat both chocolates in separate bowls in the microwave in 30-sec blasts, or over pans of simmering water. Pour the dark chocolate into the baking tin, then pour the milk chocolate on top in a trail. Spread the chocolate out to make a reasonably thick slab.
Arrange the red smarties over the slab at intervals – these are the reindeer noses. Cut a marshmallow in half and press above the smarties to make eyes. Snap a pretzel in half for each set of antlers, snapping off any excess pretzel, then push into the chocolate near the noses.
To make the snowmen, arrange the rest of the marshmallows in lines of three. Use pieces of pretzel for the arms and a piece of shoe lace for the scarf. Sprinkle over the stars and sprinkles, then leave the chocolate to set completely.
Once the chocolate is hard, use an icing pen or small brush dipped in black food colouring to add pupils to the reindeer eyes, and faces and buttons for the snowmen. Use a knife to cut the bark into pieces around the reindeer and snowmen. To give them as gifts, put the chocolate shards in cellophane bags and tie with a pretty ribbon.