Combine the tequila and jalapeño in a jar, shake to combine, and let sit at room temperature (preferably in a dark place) for 6 – 24 hours. The longer it sits, the spicier it will get. Strain the tequila through a fine mesh strainer and discard the jalapeño and seeds. You can make a lot and keep it refrigerated for up to a year if in a covered jar or bottle.

Drain the juice from the pineapple, saving ¼ cup of the juice. Reserve 1 pineapple ring for garnish, if desired.

In a blender, combine the strained tequila, the pineapple rings, ¼ cup reserved pineapple juice, the lime juice, agave, Cointreau, and ice. Blend well.

Garnish the margaritas with a quarter of a pineapple ring and a slice of jalapeño, if desired.