Soak the cashews in water overnight, or at least 3 hours. Drain cashews and pat dry.
Add the cashews to a food processor and process until a thick paste has formed. You'll need to stop the food processor and scrape the sides several times - this will take a couple of minutes!
Leaving the food processor on, slowly stream the pure maple syrup, lemon juice, and almond milk through the opening. Add the pure vanilla extract and sea salt, and continue processing until the frosting is smooth and creamy. Taste the frosting for flavor and add more pure maple syrup and/or lemon juice if desired. For thinner frosting, add more almond milk
Transfer frosting to a sealable container or jar and refrigerate until ready to use. Frosting will keep for up to 1 week in a sealed container in the refrigerator.