Thinly slice 1 jalapeno (remove the seeds for less heat). Combine with 2 1/2 cups chopped pineapple in a shallow dish; freeze until firm, at least 3 hours. Transfer to a blender; add 3/4 cup rum, 2 to 3 tablespoons sugar, the juice of 1 lime and 1 1/2 cups ice and puree until smooth. Divide among 4 glasses; top with jalapeno slices and a cilantro sprig.
Photograph by Charles Masters