Muddle 4 of the cranberries and 1 of the rosemary sprigs with the maple syrup in a shaker. Add the gin, cranberry juice and vermouth and vigorously shake with ice. Strain into a chilled coupe. Top with a splash of Champagne.
To garnish, place the remaining 2 rosemary sprigs into the marshmallow to create antlers, pushing one in far enough to come out of the opposite side. Attach the remaining cranberry to the end that is sticking out for the nose, then place the 2 cloves between the nose and antlers to create eyes.