Soak the cashews: Place the cashews (and date, if using) in a medium bowl and add 2 cups of the water. Set aside uncovered at room temperature for 10 to 12 hours. If you are in a hurry try using warm water and soak for at least an hour. Know that the longer you soak them, the creamier they will become. The cashews are ready when they break apart when pressed between two fingers.
Drain the cashews and rinse under running water. Add the rinsed cashews (and date, nutritional yeast, or lemon, if using), remaining 1/2 cup water, and salt to a blender.
Blend on high speed in your blender until completely smooth, (approx 3 minutes). Stop scrape down the sides of the blender and process again for 1 minute.
Use immediately or transfer to an airtight container and refrigerate for up to 1 week.