Preheat oven to 350°F (175°C).
Line a 9x13 inch baking pan with foil, allowing excess to hang over edges. Spray with cooking spray.
In a bowl, combine the pretzels with the melted butter. Press into the prepared pan. Set aside.
Place the butter and the chocolate chips in a bowl and heat in a microwave in 30 second intervals, stirring after each one, until smooth.
Stir in the Nutella and cocoa powder until combined. Add the sugar, brown sugar, vanilla extract, and salt. Stir to combine. Stir in the eggs, one at a time. Stir in the flour until just combined. The batter should be thick.
Spread the batter over the crust. Bake for 30 minutes or until a toothpick inserted into the center comes out clean. Allow to cool before frosting.
In a bowl, beat together the butter and Nutella until smooth. Add the cocoa, sugar, and heavy cream, and beat once again until smooth and fluffy. If it is too thick, add more heavy cream, 1 tablespoon at a time.
Spread the frosting over the top, sprinkle with pretzels and enjoy!