Make the simple syrup: To a small saucepan add the water and sugar over medium heat. Heat until the sugar dissolves, stirring often. Remove and allow to cool. Omit this step if using store bought.

To a rocks glass/tumbler add the basil leaves, whiskey and 1 ounce (2 tablespoons) simple syrup. Use a muddler or the end of a wooden spoon to mash the basil into the liquid.

Fill the glass with crushed ice and add a splash of water. Garnish with basil leaves.