Wash the watermelon well under running water. Cut off its top with a sharp knife and save it for later. Using a spoon scoop out its insides, remove the seeds and transfer to a small pot.
In a separate container, mix the white wine, honey, lemon juice, powdered vanillas and stir until the honey (or brown sugar) dissolves completely. Mix with the watermelon insides and blend the mixture.
Return the blended mixture to the scooped out watermelon, put its cap on and transfer to a suitable tray that can fit the whole watermelon so that it can sit stably in the freezer. Cover with plastic wrap on top and put in the freezer. Stir the mixture slightly every 30 min. until it turns to fine crystals.
Serve the sorbet in the watermelon itself, garnished with fresh mint leaves. Since the watermelon rind freezes as well, the sorbet will remain cold for a much longer time, making it perfect for consumption on the beach or at a picnic.