Add remaining butter, egg, brown sugar, cinnamon, salt and nutmeg to mixing bowl.
Use an electric mixer and whip together until smooth.
Pour 1/2 of the sweet potato mixture into greased casserole dish and spread evenly.
Sprinkle the grapenuts and brown sugar evenly over the sweet potato mixture.
Gently spoon on the rest of the sweet potato mixture to cover filling and spread evenly to cover layer.
Lock on Lid and close Pressure Valve. Cook at High Pressure for 15 minutes.
When beep sounds, wait 10 minutes and then release the rest of the pressure.
Remove lid and unplug pressure cooker.
Add marshmallows to the top of the Sweet Potatoes
Place Mealthy CrispLid over pressure cooker cooking pot. Set temperature to 300 degrees and time to 4 minutes. Alternatively, place casserole dish into oven and cook at under low broiler for 1-2 minutes, or until the desired color is reached.
Layer ingredients into a 2 quart slow cooker and cook on LOW for 5 hours. Add marshmallows and cook an additional 30 minutes.
Preheat oven to 350 degrees and cook for 30 minutes. Add marshmallows and place under LOW broiler until the desired color, about 1-2 minutes.
Air Fryer Method
Cook at 275 degrees for 20 minutes. Add marshmallows and cook at 250 degrees for 8 minutes more.
Use a 7 inch casserole dish (1.5 liters) for 6 quart or larger pressure cookers. Use a 7-9 inch casserole dish for 8 quart or larger pressure cookers and double the recipe for the 8 or 9 inch casserole dish.