Add macadamia nuts and cashew pieces to the bowl of your food processor.
Process for about 5 minutes total, stopping to scrape the sides once or twice, until the butter is real smooth and creamy.
Add salt, ghee, vanilla and maple syrup and resume processing until just incorporated.
Remove blade, gently mix in Fleur de Sel with your rubber spatula and then transfer your finished butter to a half pint glass jar.
Let your nut butter rest for 24 hours to allow flavors to fully develop and texture to get where it's supposed to be.