Cranberry simple syrup gives this frozen cocktail a festive crimson hue -- perfect for the holidays!
Make the Whole-Berry Cranberry Simple Syrup: Stir together sugar, cranberries, lime zest and 2 tablespoons water in a small saucepan. Cook over medium-high, stirring constantly, until cranberries just start to burst, about 5 minutes. Remove from heat; let cool completely. Refrigerate, covered, until cold, at least 1 hour and up to 1 week.
Make the cocktail: In a blender, combine tequila, lime juice, 4 cups ice, and 1 1/2 cups cranberries and syrup from Whole Berry Cranberry Simple Syrup. Blend until smooth. Divide among 4 margarita glasses.