In a large glass, muddle the watermelon with two basil leaves to release as much juice as possible. Strain through a fine sieve and reserve the juice.

To a cocktail shaker filled with ice, add 2.5 ounces of the watermelon juice, vodka, and St. Germain. Shake vigorously. Strain into a martini glass and garnish with a watermelon slice and basil leaf.