Combine everything except for the cashew butter in a saucepan and stir everything together with a whisk. Keep over high heat constantly whisking until the mixture starts to steam, but not boil - you don't want it to scorch. Slowly add cashew butter, whisking the whole time (add it about 2 tablespoons at a time, making sure what you just added is mixed in before you add more cashew butter). After cashew butter is added and mixture is smooth, reduce heat to low, allowing mixture to simmer for about 30 minutes. Whisk regularly during this time to make sure that the mixture does not become curdled.
This is a labor intensive sauce - your arm may get tired but it's well worth it! :)