Place the water sugar and peppercorns in a saucepan. Set over high heat and stir until the sugar is dissolved. Remove from the heat and leave to cool. Meanwhile put the lemon juice vodka and salt into a shallow plastic box. Cover and chill. Once the peppercorn syrup has cooled strain into the lemon juice mixture and season to taste with freshly ground black pepper. Place in a freezer. After 2 hours remove and mix with a fork. Return to the freezer and repeat the process every hour until a slushy mixture has formed. Serve in shot glasses.
