Nutella Fudge Crumble

I got this recipe from Eat My Cake Now -- I like it and think it turned out pretty well, after a minor adjustment or two. Also, this may be obvious, but you definitely need to like nutella for this. If you don't, I'd recommend swapping it for some sort of thick fudge sauce or something similar. But the cookie base and topping is very light and buttery and delicious, and the whole thing is pretty amazing served warm with vanilla ice cream.

240 g (2 cups) all-purpose flour
160 g (2 cups) old-fashioned (rolled) oats
270 g (1 1/2 cups) brown sugar
5 ml (1 teaspoon) baking powder
1 ml (1/4 teaspoon) salt
200 g (1 cup) butter, melted
600 g (21 ounces) Nutella

1/3 cup butter, melted

Preheat oven to 350°F(180°C).
Butter a 9-by-13-inch baking pan.
Whisk together the flour, old-fashioned oats, brown sugar, baking soda and salt.
Pour the melted butter and stir to incorporate the butter.
Set aside 1½ to 2 cups of the mixture, then turn the remaining dough into the buttered pan.
Gently press the dough over the bottom of the pan.
Pour the Nutella over the oatmeal crust, then scatter the remaining oatmeal mixture over the top.
Bake for 15 min, then drizzle 1/3 melted butter over the top of bars as evenly as possible.
Bake for another 10 to 15 minutes, or until the topping is golden brown.
Transfer the baking pan to a rack and cool for about 1 hour.