In a small plate or bowl, combine 1 tablespoon granulated sugar and the zest of 1 lime. Mix together. Cut a small wedge of lime and rub it around the rim of the glass to moisten. Dip the glass into the lime sugar coating the rim. Set aside
In the carafe of a blender, combine the tequila, triple sec, and the juice of 2 limes. Add the vanilla ice cream and blend until smooth. If needed, add a little milk until you reach a pouring consistency.
Pour the milkshake into the prepared glasses. Top with whipped cream and additional lime zest. Serve immediately.
Notes
As with any recipe, good quality ingredients can make all the difference in the success or failure of the dessert. For instance, budget ice cream can be very light and fluffy, almost airy, while the better, more expensive brands are thick and rich. Good vanilla ice cream (such as Haagen Dazs brand) will make a thicker, richer milkshake. Use whatever you have but adjust ingredients as needed. Nutrition information is calculated using an ingredient database and should be considered an estimate. When multiple ingredient alternatives are given, the first listed is calculated for nutrition. Garnishes and optional ingredients are not included.