In a small shallow bowl or rimmed saucer combine the lime zest and salt and mix well. Run the lime slice around the rim of a stemmed, wide mouthed glass to moisten it, then turn the glass upside down and press the rim in the salt mixture. Set glass aside.
In a cocktail shaker combine all remaining ingredients and shake well to chill. Strain into the prepared glass and serve immediately.
Yield: 1 large margarita cocktail