Make the rosemary simple syrup: Stir together sugar, 1/2 cup water, and rosemary in a small saucepan. Bring to a gentle simmer over medium-low; cook, stirring occasionally, until mixture is syrupy and rosemary has wilted, 8 to 10 minutes. Remove from heat, and let cool completely, about 20 minutes. (If making ahead of time, remove rosemary sprigs, and store syrup in an airtight container in the refrigerator.)
Prepare your glass: Combine granulated sugar and kosher salt on a small plate. Using your fingers, rub rosemary leaves into sugar mixture until fragrant and oils release, about 30 seconds. Rub rim of rocks glass with lime wedge; dip into sugar mixture. Fill glass with ice and set aside.
Make the cocktail: Combine tequila, cranberry juice, orange liqueur, lime juice, and rosemary simple syrup in a cocktail shaker filled with ice. Shake until well chilled, about 20 seconds.
Strain cocktail into prepared rocks glass. Garnish with rosemary sprig and fresh cranberries, if using.
To batch this cocktail in advance, stir together 1 1/2 cups tequila, 3/4 cups cranberry juice, 1/2 cup orange liqueur, 1/2 cup lime juice, and 1/2 cup Rosemary Simple Syrup in a large pitcher. Chill in the refrigerator for 45 minutes. While the drink can be poured directly into the serving glass, shaking each serving in a cocktail shaker with ice will ensure a frothy, creamy texture. Serves 8.
