Preheat oven to 350 degrees F. Line an 8x8" or 9x9" square pan with foil, extending the sides of the foil over the edges of the pan. Spray the foil liberally with cooking spray. Set aside.
In a large microwaveable bowl, melt the chocolate chips and butter together on HIGH power for 45 seconds. Stir, then microwave for another 25 seconds. Stir again until any large chocolate pieces are broken up and the butter is melted and everything is smooth. Set aside to cool for about 5-10 minutes.
Add in the sugar and stir to combine. Add in the eggs and yolk, as well as the Nutella and vanilla, stirring well. Lastly, add in the flour, cocoa powder and espresso powder and salt, if you're not using salted butter. Stir until a soft batter has formed.
Spread the batter evenly into the prepared pan and bake for approx. 35-40 minutes or until a toothpick inserted near the center comes out clean or with moist, not wet, crumbs. Try not to over bake as these are intended to be fudgy and will continue to set up as they cool. Cool completely before cutting into squares.