The original recipe called for a 9 x13 pan which i didn't have. Double the recipe if you do have one.
You can store in an airtight box in the fridge or room temperature for few days but is best eaten at room temperature.
Too less nutella will give you a Nutella flavoured cookie bar and not a "stuffing",
I just used a whisk and had no trouble at all. Use a beater if you want.
Heat the nutella in the microwave or saucepan to make it easier to spread.