Preheat oven to gas mark 6.
Pop the sweet potatoes in a large bowl and drizzle with the oil and melted butter.
Add the parmesan cheese, garlic, brown sugar, cumin, smoked paprika, cinnamon, cayenne, salt and pepper.
Toss with your hands to coat.
Spread the fries over 2 trays (at least) in a single layer.
Bake for 15 minutes, then check. You could turn the fries over at this stage (i don't).
Cook for a further 15 minutes on gas mark 4 checking at 10 minutes to make sure they aren't catching.