Combine mezcal, grapefruit juice, lime juice, and jalapeño syrup in a cocktail shaker with ice. Moisten the rim of a 10-ounce rocks glass with lime juice and dip into smoked salt. Shake cocktail vigorously, pour into glass and garnish with jalapeño slice.
Jalapeño-Infused Agave Syrup:
In a small saucepan, cook the jalapeños in the agave syrup on low heat for approximately 30 minutes. Remove jalapeños and cool.