Add the cashews to a high-powered blender or food processor along the milk, salt, lemon, maple syrup, cinnamon and vanilla (if using vanilla extract, start with just 1 teaspoon). Blend on high speed until smooth, thick and creamy, scraping down the sides with a silicone spatula as you go. This will take about 2 minutes in a high-powered blender like a Vitamix, or about 4 minutes in a food processor.