Add tequila, triple sec, lime juice and simple syrup to a cocktail shaker.
Fill the shaker with ice and shake for a few moments.
Double-strain the contents into a rocks glass rimmed with salted plum powder, then garnish with the salted plum on a cocktail skewer.
Notes
Make a simple syrup by bringing equal parts sugar and water to a boil in a saucepan. Cook for 2 minutes, until all the sugar has dissolved, then let cool completely. Store, covered, in the refrigerator for up to 1 month.