Prepare the glass: Run a lime wedge around the rim of a glass and dip it into Tajín (or salt) to coat the edge. Set the glass aside.
Muddle jalapeño: In a cocktail shaker, add 1-2 slices of fresh jalapeño. Muddle gently to release the heat from the pepper without breaking it up too much.
Mix the ingredients: Add tequila, lime juice, orange liqueur, and agave syrup to the shaker. Fill the shaker with ice and shake vigorously for 15-20 seconds until well chilled.
Strain and serve: Strain the mixture into the prepared glass over fresh ice.
Garnish: Add a lime wedge and an extra jalapeño slice for garnish.