Rim a rocks glass with Tajín or chili-lime salt. Set aside.
Add jalapeño slices to a cocktail shaker and press gently with a muddler.
Pour rum, lime juice, triple sec, and agave nectar into the shaker.
Fill the shaker with ice and shake hard for 10-12 seconds.
Double strain the mixture through a Hawthorne strainer and fine mesh strainer into the prepared rocks glass filled with fresh ice.
Garnish with a lime wheel.
Notes
The spice level can be adjusted by using more or fewer jalapeño slices. Remove the seeds for less heat.Fresh lime juice makes a big difference - avoid bottled juice.The drink can be made sweeter by adding more agave nectar in 1/4 oz increments.Store leftover spiced rum at room temperature in a sealed bottle.