In a small bowl, combine the kosher salt, chile powder and lime zest.
Moisten the rim of two glasses (or mason jars because they’re festive) with a lime wedge. Gently dip the rim of each glass in the chile salt and twist to coat. Briefly set aside while you whip up your margs.
Place the tequila, grapefruit juice, lime juice and agave nectar in a cocktail shaker with ice. (If you don’t have a cocktail shaker, relax. You can just mix the ingredients in a large glass.) Shake vigorously and pour over ice into each of your rimmed glasses and serve!