To make these Nutella Brownies, I used the recipe from Nutella: les 30 recettes culte and adjusted it just a bit.
First, preheat the oven to 390F. Butter and flour the pan. Melt 150g of butter in a small pot, then add 100g of Nutella and mix well. Remove the pot from the stove and add 100g of dark chocolate. Mix well. In a separate bowl, beat 3 eggs, 100g of sugar, and 2 packets of vanilla sugar until light and fluffy. Add the chocolate mix, 100g of flour, and a teaspoon of baking powder in slowly. If you want to, you can add about a handful of chocolate chips. Mix the batter gently until just combined. Bake for about 20 minutes.