Place the cubed watermelon in a blender and pulse until smooth, about 20–30 seconds. Strain the purée through a fine-mesh sieve into a bowl, pressing solids to extract as much juice as possible. You should have roughly 1 cup of watermelon juice.

Make syrup

In a small saucepan combine the watermelon juice, granulated sugar, and 1/2 cup water. Add the sliced jalapeño (include seeds for more heat, remove for milder syrup). Bring to a gentle simmer over medium-low heat, stirring until the sugar fully dissolves, about 3–4 minutes. Do not boil vigorously — just a simmer to infuse flavor.

Infuse and finish

Remove the pan from heat and let the syrup infuse 15–20 minutes, tasting after 10 minutes; remove jalapeño earlier if it becomes too spicy. Once the desired heat is reached, stir in 1 tablespoon fresh lime juice and optional lime zest. Strain the syrup through a fine-mesh sieve into a clean jar, pressing solids to extract liquid. Cool to room temperature, then refrigerate. Syrup will keep 7–10 days refrigerated.

Cocktail Assembly

Prepare glass

If you like, rim your serving glass: rub a lime wedge around the rim and dip into coarse salt or Tajín. Fill the glass with fresh ice and set aside.

Shake

Add 2 oz Espolòn Tequila Blanco, 0.75 oz fresh lime juice, 0.75 oz chilled watermelon & jalapeño syrup, and 6 units of ice to a cocktail shaker. Shake vigorously for 12–15 seconds until the shaker is frosty and well chilled.

Strain

Double-strain the mixture (using a Hawthorne and fine mesh strainer if available) over fresh ice in the prepared serving glass.

Garnish and serve

Garnish with a thin watermelon wedge and an optional thin jalapeño slice placed on the rim or floated on top. Serve immediately.