In a pitcher, combine the lime zest, lime juice, Cointreau and tequila. Taste the base, if you like the bracing flavor (it will be quite tart), leave the mixture as is; if it is to tart for you stir in a little superfine sugar. Cover and refrigerate until chilled, about 1 hour.
To serve, strain the mixture to remove the zest. Moisten the rims of 5 martini glasses with the cut side of the lime half. Spread coarse salt on a small plate, and then dip the glasses into the salt to crust the rims.
Fill each glass half way with the margarita base (it will take 1/3 cup). Slowly fill the rest of the glass with Champagne or sparkling wine.
Serve with nachos (lightly salted) and guacamole.
For the guacamole:
Place the avocado flesh in a medium bowl and use a fork to mash until almost smooth. Add the onion, tomato, garlic, chilli and lime juice and use a spoon to mix well. Taste and season with salt and pepper.
Recipe by Executive Chef Tiaan van Greunen. Try it for yourself on Wednesday evenings from December until the end of February 2013 at Reuben’s at the Robertson Small Hotel.