To a small saucepan add water and sugar. Heat over medium heat until the sugar dissolves, stirring often.
Remove the pan from heat and allow the syrup to cool completely. Omit this step if using store-bought syrup.
Make The Cocktail
To a large serving glass, add basil leaves, whiskey, and 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 top it with chilled water. Stir well using a stirrer or a bar spoon.
Check for sugar and add more syrup if you like your drink sweeter.
Garnish with basil leaves and serve immediately.
Video
Notes
The crushed basil leaves will keep floating in the drink. You can strain the drink if you don't want the leaves to come into your mouth while sipping.The syrup is good to make 3-4 glasses of the cocktail. You can store the leftover sugar syrup in an airtight container in the refrigerator for up to 15 days.