Add the tequila, lime juice, Cointreau, and simple syrup to a cocktail shaker filled with ice. Shake and double strain into the rimmed glass.
2 ounces (60 milliliters) tequila, ½ ounce (15 milliliters) Cointreau or other triple sec, ½ ounce jalapeño simple syrup, ¾ ounce (22 milliliters) freshly squeezed lime juice