If desired, rim a glass with lime juice and dip into Tajín or salt. Set aside.
In a cocktail shaker, muddle the jalapeño slices to release flavor.
Add tequila, Cointreau, lime juice, pineapple juice, and a handful of ice to the shaker.
Shake vigorously for 15-20 seconds until well chilled.
Strain into the prepared glass filled with ice.
Garnish with a pineapple wedge and jalapeño slice. Serve immediately.