Muddle 3 slices cucumber, 2 tablespoons cilantro leaves, the sugar and 1 lime wedge in the bottom of a cocktail shaker. Add the tequila, triple sec and lime juice. Fill the shaker with ice and shake heavily. Rub a rocks glass with the remaining 2 lime wedges and coat with salt (optional). Fill a glass with ice and strain the cocktail into the glass. Garnish with the cilantro sprig and remaining cucumber slice.