Clap the mint leaves between your hands to release the fragrant oils, and drop into a cocktail shaker. Add the gin, syrup, lemon and lime juices, and 1 cup of ice and shake until the outside of the shaker is very cold, about 25 seconds. Fine strain into a coupe (to strain out all small ice shards) and garnish with a sprig of mint and mojito berries on a toothpick, resting on the rim of the glass.