Place the basil leaves in a cocktail shaker, add the simple syrup and lime juice. Muddle the basil leaves with a muddler. You can also use the bottom of a large wooden spoon. Pressing into the basil leaves and liquid helps to release the oils of the basil and all the flavors!
Fill the shaker with a good amount of ice and pour your favorite spirit along with the St. Germain Elderflower Liqueur over the ice. Shake well for about 30 seconds.
Using a cocktail strainer, pour the mixture into a cocktail glass. Garnish with a sprig of Thai Basil.