Combine the watermelon puree, cream, milk, and sugar and stir until the sugar has dissolved. Stir in lemon juice. Freeze in ice cream machine according to manufacturerâ€™s instructions. Once frozen, stir in chocolate chips. Pour into the center of the lemon sherbet thatâ€™s already been pressed into a bowl. Place to freezer to harden. Once firm, flip onto a serving platter. Slice and serve.