How to Make It
Simmer sugar, water, and zest together until the mixture thickens, and let it cool to room temperature. Stir in lime juice. Strain and pour this into a lidded jar. Any excess can be stored in the refrigerator for up to a week.
Pour ¼ cup of the lime syrup into a pint glass, then one cup of cold brew coffee. Stir, and add sparkling water, tasting as you go. Add plain ice or frozen limeade that you cleverly crafted from stirring some of the lime syrup with water to taste and pouring into an ice cube tray. Garnish with a lime slice if you're feeling sassy.
Should you care to lighten the mixture, stick with non-dairy milk to avoid curdling.