Watermelon-Lime Frozen Agua Fresca

• MAKE-AHEAD

Ingredients

1. 1/2 large seedless watermelon, rind removed, flesh cut into 1-inch dice (10 1/2 cups)
2. 1/2 cup plus 1 tablespoon fresh lime juice (5 limes)
3. 1/2 cup plus 1 tablespoon sugar
4. 1/4 cup plus 2 tablespoons dark rum, preferably Jamaican

1. Put 9 cups of the watermelon cubes on a rimmed baking sheet, wrap with plastic and freeze until solid, about 1 hour. Meanwhile, freeze 6 highball glasses. Chill the remaining 1 1/2 cups of watermelon.
2. In a blender, combine half of the chilled watermelon with half of the frozen watermelon and half of the lime juice, sugar and rum. Blend until smooth. Pour into 3 of the frozen glasses. Repeat with the remaining chilled and frozen watermelon, lime juice, sugar and rum. Serve the agua fresca at once.