1. Combine first 3 ingredients and 1/2 cup tequila in a medium saucepan over medium heat; cook 3 minutes or until tiny bubbles form around edge of pan. Remove tequila mixture from heat; steep 15 minutes. Strain through a sieve into a bowl; discard mint. Stir in juice, Grand Marnier, and remaining 1/4 cup tequila. Cool to room temperature. Add bitters, if desired.
2. Place mixture in a zip-top plastic bag; seal and freeze 2 hours (alcohol will separate). Knead bag to combine mixture. Pour 2/3 cup margarita into each of 6 margarita glasses.