Prep your glass. Run a lime wedge around the rim of a rocks glass. Dip the rim into salt or tajín.
Set aside with fresh ice.
Muddle the jalapeño. In a cocktail shaker, add 3–4 jalapeño slices. Gently muddle 2–3 times to release flavor without pulverizing the peppers. Overdoing it can make the drink bitter.
Add the liquids. Pour in the tequila, fresh lime juice, and agave syrup.
If you like a rounder flavor, add a small splash (1/4 oz) of orange liqueur.
Shake with ice. Fill the shaker with ice and shake vigorously for 10–12 seconds. You want it cold and slightly diluted for a smooth sip.
Strain and serve. Double strain into your prepared glass over fresh ice to catch jalapeño bits and seeds.
Garnish. Add a fresh jalapeño slice or lime wheel. For a cooling contrast, tuck a cucumber slice on the rim.
Taste and adjust. If it’s too spicy, add a touch more agave or an extra splash of lime.
If it’s too tart, bump up the syrup slightly. For more heat, float a thin jalapeño slice on top.