Preheat your oven to 350°F (175°C). Grease and line an 8×8-inch baking pan with parchment paper, allowing some overhang for easy removal later.

1. In a large bowl, whisk together the melted butter and granulated sugar until smooth. Add the eggs and vanilla extract, and whisk until fully incorporated. Sift in the flour, cocoa powder, salt, and baking powder, and stir until combined.
2. Pour the brownie batter into the prepared baking pan, spreading it out evenly.
3. In a medium bowl, beat the softened cream cheese and sugar together until smooth and creamy. Add the egg and vanilla extract, and beat until combined. Gently fold in the Nutella until the mixture is smooth and swirled, but not fully incorporated.
4. Spoon the cheesecake filling on top of the brownie batter in dollops, then use a knife or skewer to swirl the Nutella cheesecake filling into the brownie batter, creating a marbled effect.
5. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out mostly clean (a few moist crumbs are fine). The cheesecake filling should be set, and the brownies should have a slightly cracked top.
6. Allow the brownies to cool in the pan for at least 10 minutes before transferring them to a wire rack to cool completely. For the best texture, refrigerate the brownies for at least 1-2 hours to allow the flavors to meld.
7. Once chilled, lift the brownies out of the pan using the parchment paper, slice into squares, and serve. Enjoy the delicious layers of Nutella cheesecake and fudgy brownie!