In a cocktail shaker, combine the gin, rich simple syrup, lemon juice and 2 to 3 pieces of crushed ice. Shake vigorously, then pour ½ ounce of the mixture in the bottom of a rocks glass.
Add 4 of the cranberries to the glass and muddle well. Fill the glass with crushed ice and pour the remaining cocktail into the glass.
Top with even more crushed ice then garnish with the remaining cranberries. Dust the cranberries with confectioners' sugar, then garnish with a mint sprig and serve.