You can cut the potatoes upto 24 hours in advance. They will darken a bit though. To avoid that, place them in a bowl full of water. You can also prep everything for the hasselback potatoes, including the brushing them with butter and cover it with foil. Do the roasting just before serving. I have also, successfully reheated cooked hasselback potatoes again in the air fryer the next day without any loss of taste.