Line an 8-inch square baking dish with parchment paper, leaving overhang on both sides. Scoop the Nutella into the pan and spread evenly into the pan. Freeze for at least 1 hour, or until solid.

Using paper overhang, remove Nutella slab onto a plate, peeling the parchment paper off of the slab. Return Nutella slab to the freezer and place the parchment paper back into the pan. Spray pan with nonstick cooking spray.

Preheat oven to 350°F.

Place the butter and chocolate into a large microwave-safe bowl. Heat in the microwave for 1 minute, remove, stir, then return to the microwave for another 30 seconds. Stir mixture until smooth and silky. If mixture still has lumps, continue microwaving at 30-second increments and stirring until completely smooth.

Add both sugars to the butter mixture and whisk vigorously until combined. Allow to cool until just barely warm.

Add the eggs, yolk, and vanilla, and whisk until smooth.

Using a rubber spatula, fold in the flour, cocoa powder, salt and espresso powder until just combined.

Pour half of the batter into the prepared pan and spread out into an even layer. Carefully place the frozen Nutella slab on top of the batter. Pour remaining batter on top, covering Nutella slab completely.