Combine the lime juice, simple syrup, lychees and their syrup and jalapenos in a cocktail shaker and mash with a muddler until the lychees are broken down. Bruise the cilantro with your hands, and add it to the shaker along with the tequila. Fill the shaker with ice and cover and shake vigorously until very cold.
Pour the drink into a double rocks glass and serve immediately.
Recipe courtesy of Shawn Thomas, The Tar Pit