Ok, I used a 10.5x7. I sprayed it liberally with nonstick cooking spray and then got to the important part, the mixing! Preheat your oven to 350 degrees F.
In a bowl mix...
1 1/4 cups Nutella
1/2 cup flour
(I added 3/4 cup Heath toffee bits) OPTIONAL
Mix all ingredients except toffee bits until completely combined. Pour into your prepared backing dish and although it says to bake for 15 minutes, mine took nearly 30-35. You can see they were not thick at all, so I am not sure how that timing is off so much but for me, it was. Maybe for you it will be done at 15 minutes. Doubt it, but maybe. Let cool completely before cutting and serving.
I love Nutella so nothing about these was bad, NOTHING. You will only need small piece they are in fact very rich! You could mix in, or stir in darn near anything, let me know if you do!