Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking pan.
In a large bowl, mix the melted butter and granulated sugar until combined.
Add the eggs one at a time, mixing well after each addition. Stir in the Nutella until fully incorporated.
In a separate bowl, whisk together the flour, cocoa powder, salt, and baking powder. Gradually add this dry mixture to the wet mixture, folding gently until just combined.
Pour the batter into the prepared baking pan and bake for 25 to 30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
Allow brownies to cool completely in the pan before cutting into squares and serving.
Notes
For extra decadence, serve warm with a scoop of vanilla ice cream.