On a small plate mix tamarind candy and salt. If not using the candy simply spread salt on plate. With lime wedges moisten rim of 6 ounce glasses. Dip glasses into salt or candy mix. Place glasses into freezer to chill.
In a pitcher combine tequila, agua de tamarindo, lime juice and Cointreau. Remove glasses from freezer and serve margarita mixture over ice. Garnish with lime slices.