Add 1 cup water, 1 cup sugar, and 1 cup mint leaves to a small saucepan. Bring to a boil over medium-high heat. Remove from heat and steep for an hour. Strain into a bottle or jar and cool completely before using. The syrup can be stored in the fridge for several weeks.
Extra Minty Mojito
Muddle together 1 ounce mint simple syrup, 1/2 small lime, and 6 large mint leaves (make sure you do this in a very sturdy tall glass or use a cocktail shaker or measuring cup). Stir in 1 1/2 ounces rum. Fill the glass halfway with ice (or fill a tall glass halfway with ice and transfer your muddled mixture-rum to the glass). Top with club soda (use lime-flavored soda if you like your mojito extra limey). Garnish with additional mint leaves or lime wedge as desired.