Add mint leaves and simple syrup to the base of the cocktail shaker and muddle until fragrant, about 30 seconds. Add rum and lime juice, then fill the shaker with ice. Shake the mixture until just combined and chilled, about 5 seconds.
Double-strain cocktail into a chilled collins glass half-filled with ice. Add seltzer and using a bar spoon, gently lift rum mixture from the bottom of the glass to top to combine. Top with additional ice and garnish with a mint sprig. Serve & Enjoy!
(From America's Test Kitchen's How to How Cocktail)