In a saucepan, combine the sugar and the water. Heat the saucepan over medium heat, stirring occasionally, until the sugar is completely dissolved and the syrup just comes to a simmer.
Lightly bruise the fresh basil leaves by crushing them lightly in the hand and then add to the syrup. Turn off the heat and cover. Let steep for 20 minutes. Strain the syrup and cool completely before using.
Limoncello Basil Fizz
In a cocktail shaker, add the Limoncello, basil simple syrup, fresh lemon juice, and 3-4 fresh basil leaves. Add ice, cover, and shake for 30 to 45 seconds.
Strain and divide the cocktail between two coupe glasses. Top each glass with 3 to 4 oz of Prosecco and enjoy!