Slice half the lime into 3 rounds. Make a slit in one of the slices of lime and slide it around the rim of a rocks glass to moisten. Put an equal amount of Tajin and hot pepper flakes on a plate, then roll the wet rim of the rocks glass in the chile pepper mixture so that it sticks to the rim. Set the lime slice aside for garnish.
Put two more slices of lime with as much tajin and hot pepper flakes as you can handle in the bottom of the rocks glass and muddle to release the flavors.
Fill a cocktail shaker with ice. Add the whisky, simple syrup and the juice from the remaining lime. Shake vigorously until the outside of the shaker becomes frosty then pour the contents into a rocks glass. Garnish with the saved lime round and the dried chile pepper and serve immediately.
