Tried and tested; All you need is 2 eggs, 350g nutella and 150g flour. Mix them together until it forms a rather sticky dough. Place in a dish, and put in a preheated oven, and cook at 160 degrees Celsius. It took mine 25mins to be fully cooked, but always use a toothpick to check if the cake is ready. As soon as toothpick comes out of the cake clean, switch the oven off and wait for the brownie to cool down slightly before eating it.
Below, you can find a photo of the end result. I have added almonds and chocolate chips on top of my cake: