Add ice to a shaker with tequila, fresh lime juice, triple sec, Firefly tonic or brine and simple syrup (if using). Put on lid and shake for about 1 minute, until very well mixed.

Use one of the juiced limes to run around the rim of two 12- to 16-ounce glasses of your choice. Dip in Firefly’s Cortido Salt to rim the top of the glass. Fill glasses with ice and split cocktail base between the two glasses.

Top each with 4 to 6 ounces of sparkling mineral water, give them a quick stir and serve with a lime slice or wedge.