Preheat oven to 350°F (176°C). Grease or line a 9×9 inch baking pan with parchment paper.
In a bowl, mix melted butter, sugar, and vanilla extract until combined. Add eggs and whisk until smooth and glossy.
In another bowl, whisk together flour, cocoa powder, baking powder, and salt.
Add dry ingredients into the wet mixture and stir until just combined. Do not overmix.
Pour batter into prepared pan and spread evenly.
Bake for 18–20 minutes or until a toothpick comes out with a few moist crumbs. Let brownies cool completely.
To make the fudge layer, combine sweetened condensed milk, butter, chocolate chips, and Nutella in a heatproof bowl over simmering water.
Stir constantly until smooth, thick, and glossy.
Pour fudge over cooled brownies and spread evenly. Refrigerate for 20–30 minutes until set.
Melt chocolate chips and shortening in the microwave using 15-second intervals, stirring between each interval.
Spread melted chocolate topping evenly over the fudge layer.
Let topping set, then slice into bars and serve.