Slice jalapeño and remove seeds or keep a few seeds for extra heat. Muddle 2 - 3 jalapeño slices in a cocktail shaker. Add ice. Add tequila, lime juice, agave, and orange liqueur. Shake until cold. Prepare cocktail glass by rimming with a lime wedge and dipping into Tajin spice. Fill glass with ice. Strain cocktail mixture into glass. Garnish with fresh lime.