How to Make It
Prepare the Salted Jalapeño Syrup: Stir together sugar, water, jalapeño slices, and salt in a medium saucepan. Bring mixture to a boil over high, stirring until sugar and salt dissolve, 3 to 5 minutes. Remove from heat; allow to cool to room temperature, about 30 minutes. Store in an airtight container in refrigerator up to 2 weeks.
Prepare the Lime Rim Salt: Process sugar, salt, and lime zest a mini food processor until mixture is evenly blended, is a light green color, and looks like sand, 30 to 45 seconds. Transfer mixture to a dry, airtight container. Store at room temperature 3 to 5 days.
Prepare the Margarita: Dip rim of an old fashioned glass in lime juice, and then in Lime Rim Salt to coat rim of glass. Fill glass with ice cubes. Combine tequila, lime juice, Cointreau, Salted Jalapeño Syrup, and jalapeño slices in a cocktail shaker; use a wooden spoon handle or a muddler to lightly crush jalapeño slices. Add ice to shaker; place top on shaker, and shake vigorously until outside of shaker is icy, about 30 seconds. Strain into prepared old fashioned glass over ice. Garnish with a lime wheel and a jalapeño slice, if desired. Enjoy immediately.