Add rum, simple syrup, lime juice, mint, ice and seltzer (or club soda) to a blender. Blend until the ice is crushed and the mixture is slushy.
Divide between 2 Collins glasses or other tall 12-ounce cocktail glasses. Garnish with mint sprigs, if desired, and serve immediately.
Tip: To make simple syrup, simmer equal parts cane sugar and water until dissolved, then cool completely before using. Refrigerate extra simple syrup in an airtight container for up to 1 week.