Prepare the watermelon puree by blending the watermelon chunks until smooth. Strain through a fine-mesh sieve to remove pulp, which will give you a cleaner-tasting cocktail.
Prepare your glasses by running a lime wedge around the rim, then dipping into Tajin or chili-salt mix. Fill glasses with ice and set aside.
In a cocktail shaker, muddle 2-3 jalapeño slices (depending on your heat preference) until the oils are released and you can smell the pepper’s aroma.
Add 1 cup of watermelon puree, tequila, lime juice, orange liqueur, and agave nectar to the shaker with the muddled jalapeño.
Fill the shaker halfway with ice and shake vigorously for 15-20 seconds until the outside of the shaker feels cold.
Strain the mixture into your prepared glasses, using a fine-mesh strainer if you prefer no jalapeño bits.
Garnish with a watermelon triangle and additional jalapeño slice if desired. Serve immediately while cold.