Combine mint and simple syrup in a double Old Fashioned glass.

Mash the mint with the back of a spoon to release its flavor.

Add rum, sour mix, and ice to the glass.

Transfer the mixture to a cocktail shaker.

Shake well until chilled.

Serve the drink in the double Old Fashioned glass.

To make simple syrup: Combine sugar and water in a saucepan.

Bring the mixture to a boil, stirring until the sugar dissolves completely.

Cool the syrup completely before using.

Store the syrup in a covered container in the refrigerator for up to 2 months.