Rub a cut lime around the rim of each of 6 glasses. Fill a saucer with salt, and dip the glass, upside down, into the salt. Set aside. Fill a blender jar 3/4 full with ice cubes. Pour the tequila, orange liqueurs, lime juice, and simple syrup over the ice. Begin blending on low speed, and then increase the speed to high, until the ice is very finely chopped and the mixture is frothy. Taste for sweetness, adding more simple syrup, if necessary. Blend a few seconds more, and pour into the salt rimmed glasses. Serve immediately.
In a small saucepan, combine sugar, 1/4 cup water, and lime zest. Bring to a boil, stirring occasionally until the sugar has dissolved. Remove from heat, and cool to room temperature. Store in a covered container in the refrigerator for up to 3 days. Yield: 6 tablespoons
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