In a cocktail shaker, add the ¼ cup loosely packed basil leaves and gently muddle them to release the basil essence.
Add enough ice to fill the shaker or cocktail mixing glass halfway.
Pour the limoncello and vodka into the shaker or mixing glass and shake or stir well.
Strain the chilled mixture into a coupe glass or a similar glass, filling it halfway.
Top with prosecco or club soda.
Garnish glasses with 3 small basil leaves and a thin slice of lemon.
Serve immediately.