I found this one in Food & Wine. I excluded the simple syrup – seemed sweet enough for me.
1 teaspoon ancho chile powder
1 teaspoon kosher salt
1 teaspoon sugar
1/2 oz fresh lime juice
2 oz blanco tequila
1 lime wedge
2 ounces watermelon juice
Combine salt ingredients in a small bowl big enough to fit your glass into.
In a blender, purée watermelon. Dip the glasses in the foam, shake, then dip into salt mixture.
Combine remaining ingredients in a cocktail shaker. Shake. Serve over ice.
Makes 1 cocktail.
Salt for lots. I store mine in a Tupperware container for future use.