• 2 ounces of premium vodka, such as Tito's
• 1 cup chopped watermelon
• 3-4 fresh mint leaves
• 1-ounce lime juice
• 1/2-ounce Cointreau
• 1-ounce simple syrup

Instructions

1. Make your simple syrup. Bring equal parts sugar and water to a boil in a saucepan, then cool (store in the refrigerator when not in use).
2. Muddle watermelon and mint. In your martini shaker, gently muddle watermelon cubes and mint to release the juices. 
3. Add everything else and lots of ice. Add vodka, Cointreau, lime juice, simple syrup, and ice. Seal the shaker tightly and shake vigorously for about 30 seconds. The sign of a well-shaken martini is when you see ice crystals floating across the top.
4. Serve. Strain into a martini glass and garnish with a watermelon wedge or mint leaf.