Step 1: Fill two martini or margarita glasses with ice cubes and add some cold water to chill them. Place the salt in a shallow bowl.

Step 2: Cut the limes in half crosswise. If desired, grill the halves cut sides down. Dump and discard the ice and water from the glasses. Rub the rims of the glasses with the cut edge of a lime half. Dip the rim of each glass in the salt to coat it (optional).

Step 3: Squeeze the lime halves and measure 2 ounces (1/4 cup) of juice. Place the lime juice in a cocktail shaker and add the mezcal, Cointreau, and 1 to 2 teaspoons agave syrup if you like a sweeter margarita. Add enough ice to submerge the liquids by 1 inch. Shake vigorously, and I mean vigorously, for about 20 seconds. Strain the margaritas into the salted glasses and serve at once.