If you enjoy a salty spicy rim on your glass, prep the rim of your glass by rubbing a lime wedge along the rim. Dip in tajín salt and set aside.
Make the marg:
Add the jalapeño rounds (slices) with or without the seeds (the seeds add heat) inside a cocktail shaker. Add the lime juice and simple syrup (if using), and with a muddler, press down to release the jalapeño juices.
Add the tequila, Cointreau, mango juice, and a handful of ice cubes in the shaker. Shake well until the sides are fully frosted.
Serve:
Fill the prepared glass with fresh ice. Strain the spicy mango margarita through the strainer and pour it into the prepared glass. Garnish with fresh pineapple wedges and additional jalapeño slices.
Notes
Make it a pitcher (serves 8) by adding ½ to 1 sliced jalapeño to the bottom of a pitcher. Measure and pour 1 ½ cups tequila, 1 ½ cups Cointreau or triple sec, 1 cup lime juice, ¾ cup of simple syrup, and 3 cups mango juice into the pitcher. Use a wooden spoon to stir and refrigerate for 1 hour or overnight. Prepare the glasses, add ice, and serve.