Simply prepare your favorite brownie mix according to package directions. Spray a mini muffin pan (or 4) with baking spray. Place a heaping spoonful of brownie batter in the bottom of each well. Place about a teaspoon of Nutella on top and cover with another spoonful of brownie batter. Bake according to your mix's directions. I think baking cups might be helpful but I didn't have them handy. I only have one mini muffin pan so I poured the rest of my batter into a small baking dish to make regular brownies.
I wish I made this up but a friend shared this on Facebook recently. I don't think it was her idea either but I'm glad she passed it along.
Sorry, but I dug in before taking the shot. Once you try them, you won't blame me.

Update: definitely use muffin cup liners. However if they break, the brownies make an excellent ice cream topping, or so my husband says.
-Amy