Place the mint leaves in the bottom of the cocktail shaker and muddle them until you begin to smell the minty aroma.
Add the tequila, lime juice, and simple syrup to the cocktail shaker. Add a handful of ice and shake, until the sides are frosted.
Serve:
Fill a tall glass with ice. Strain the cocktail into the glass. Top off the cocktail with Topo Chico (sparkling water or club soda), and garnish with additional mint leaves.