To make the watermelon puree: Scoop out half of a small watermelon into a blender and puree on high until smooth. Strain through a fine-mesh strainer to remove the seeds and pulp. This will yield about 2 ½ cups watermelon puree.
In a cocktail shaker filled with ice, add gin, watermelon puree, lemon juice, and basil. Shake vigorously and strain into 2 champagne flutes. Top each glass with 2 ounces of sparkling wine.
Recipe by Allons-Eat! at https://www.allonseat.com/watermelon-basil-french-75/