7mlsugar syrupalso known as simple syrup (see notes)
Rub the lime slice around the rim of a glass: turn the glass upside-down and dip the wet rim into a saucer of salt.
Place ice cubes, dark tequila, Cointreau, lime juice and sugar syrup in a cocktail shaker; shake vigorously.
Strain into the prepared glass and serve immediately.
Sugar syrup: Combine 220 g caster sugar with 250 ml water in a small saucepan over a low heat. stirring until the sugar dissolves, then bring to the boil. Reduce the heat; simmer, uncovered, for 5 minutes. Cool. Store in the refrigerator, in an airtight container, for up to one month. Makes 50 ml.