1. Combine first 2 ingredients in a glass container. Crush basil gently with a cocktail muddler or wooden spoon to release flavor. Cover and set aside, out of direct sunlight, 2 days. Strain.
2. Pour 2 ounces (1/4 cup) basil-infused vodka in a highball glass filled with ice cubes. Stir in 1 tablespoon or desired amount simple syrup. Top with 3 to 5 ounces tonic water. Garnish, if desired.