Grapefruit Rum Cooler

4 mint sprigs
1 tablespoon superfine sugar, or to taste
2/3 cups fresh grapefruit juice
1½ tablespoons fresh lemon or lime juice, or to taste
3 ounces light rum (6 tablespoons), or to taste
Chilled club soda or seltzer
Angostura bitters to taste

Garnish: grapefruit wedges and mint sprigs

In a cocktail shaker with the back of a spoon crush mint sprigs with sugar and juices until sugar is dissolved. Add rum and about 1 cup ice cubes and shake 30 seconds. Strain mixture into 2 tall glasses filled with ice cubes and top each drink with club soda or seltzer and a splash of bitters. Stir drinks and garnish with grapefruit and mint. Makes 2 drinks.