Heat the sugar and water in a small saucepan over medium high heat. Stir to dissolve the sugar, and once the mixture starts to simmer, reduce the heat to low. Add the lime zest and juice, and allow the ingredients to simmer gently for 20-25 minutes, until the mixture has thickened and slightly reduced. The syrup may take on a slight golden hue, but do not let it darken too much - you do not want it to to caramelize.
Remove the lime-infused rich simple syrup from heat, and allow to cool. Strain the syrup into an airtight bottle or jar, discarding the spent lime peels. Chill for at least an hour before using for best results. A rich simple syrup can last up to 6 months in the refrigerator.
Prepare 2 coupe or lowball glasses with a sugar rim.
Combine the tequila, grapefruit juice, elderflower liquor, and lime syrup together in a cocktail shaker with two ice cubes. Shake until frosty. Strain the cocktail into the prepared glasses. Share with someone you care about (or drink them both – no judgments here!).
Preparing a Sugar Rim: I find the easiest way line a glass with a sugar rim is to first slide a lime wedge around the rim of the glass. This coats it with lime juice (but water can be used as well). Then, roll the rim of the glass in a saucer of extra fine sugar to coat. If you choose to forego a sugar rim, you may want to increase the amount of lime simple syrup in the final cocktail, depending on your sweetness preferences.