Run a fresh lime wedge around the rim of the glass. Invert the glass and dip the rim in Tajin seasoning or a combination of coarse salt and Tajin seasoning.
Fill a cocktail shaker with ice. Add the tequila, Cointreau, pineapple juice, lime juice and the Jalapeño slices. Cover the shaker with the lid and shake hard for 20 to 30 seconds.
Strain the cocktail into the rimmed glasses (with or without ice) and garnish with a wedge of pineapple or lime and a couple of Jalapeño slices.