In a blender, combine the watermelon seeds, cashews, and water and process until a smooth milk is achieved.
Pour this through a nut milk bag and squeeze to strain the milk into a pot. There will be only a small amount of pulp.
Heat the milk over medium-low heat, occasionally scraping the bottom and sides with a silicone spatula. As it heats up, tiny curds will form and solidify, with the boiling bubbles formed from the "whey."
When almost the whole mass has formed curds, stir in the salt and then pour it through a sieve or ricotta mold set over a bowl. Let the mixture drain for 30 minutes to 1 hour, or longer in the refrigerator until it is as firm as you like.
Transfer to a storage container and store in the refrigerator for up to 2 weeks.
