Preheat your oven to 350°F (175°C). Line an 8x8 inch baking pan with parchment paper, leaving some overhang on two sides so you can lift the brownies out easily.
Melt the butter and pour it into a large bowl with 3/4 cup of Nutella. Whisk until smooth and glossy, then whisk in the sugar, followed by the yogurt and vanilla, until fully combined.
In a separate bowl, whisk together the flour, cocoa powder, baking soda and salt. Add to the Nutella mixture and fold with a spatula just until no dry streaks remain, then fold in the chocolate chips.
Spread the thick batter evenly into the prepared pan, pushing it into the corners.
Microwave the remaining 1/3 cup of Nutella for 15 to 20 seconds, until loose and pourable. Drop small spoonfuls over the batter and drag a toothpick through them in loops to create swirls, then sprinkle the hazelnuts on top.
Bake for 25 to 30 minutes, until the edges are set and a toothpick inserted in the center comes out with moist crumbs but no wet batter.The center will still look a little soft when you take it out. It firms up as it cools, which is what gives these brownies their fudgy texture.
Let the brownies cool completely in the pan, about 2 hours. Lift them out using the parchment and cut into 16 squares. For extra clean edges, chill for 30 minutes before cutting.