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.