Place 2 cups cold water and 1 ½ cups granulated sugar in a 2 quart pot. Bring to a boil over medium high heat, stirring constantly until the sugar is dissolved. Boil for 5 minutes.
Remove from the heat and add the lemon juice, basil, and zest. Using a wooden spoon or other utensil, gently mash the basil leaves against the bottom of the pot to release the flavor. Do this a few times, then cover the pot and allow the syrup to steep for 30 minutes.
Strain the syrup through a fine mesh strainer into a large pitcher, pressing the zest against the strainer with a rubber scraper to get all the juice and flavor, then add the remaining 2 cups of cold water. Stir to combine.
Cover the pitcher and chill in the refrigerator for a minimum of 2 hours before serving.
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Notes
Tips for customizing basil lemonade
Make it a refreshing summer cocktail by adding 2 ounces of vodka to 6 ounces of basil lemonade
Adjust the amount of lemon juice to suit your taste
Mix 4 ounces of lemonade with 4 ounces of sparkling water
Garnish with fresh fruit such as raspberries or strawberries