01 -
Remove the rind and cut the flesh into uniform 1/2-inch cubes until you have 8 cups. Discard any seeds and blot the cubes lightly with a towel to remove surface moisture.
02 -
Place the watermelon, lime juice, lime zest, and powdered pectin into a large, heavy pot. Stir for about 1 minute so the pectin fully dissolves and coats the fruit.
03 -
Set the pot over medium-high heat and cook, stirring constantly, until the mixture reaches a full rolling boil that cannot be stirred down.
04 -
Pour in the sugar and salt all at once. Return to a full rolling boil and boil hard for exactly 1 minute. Remove from heat, skim off any foam, and stir in the vanilla if using.
05 -
Ladle the hot preserves into clean half-pint jars, leaving 1/4 inch of headspace. Let them cool to room temperature uncovered, then seal and refrigerate.