Ultra-fudgy dark chocolate brownies swirled with creamy Nutella, studded with toasted hazelnuts and dark chocolate chips, and finished with a touch of flaky sea salt for the perfect crackly top.
Preheat oven to 350°F. Line an 8-inch square pan with parchment, leaving overhang, and lightly grease.
In a heatproof bowl, melt the butter and chopped dark chocolate together in short microwave bursts or over a double boiler, stirring until smooth. Let cool for 5 minutes.
Whisk in Nutella, granulated sugar, and brown sugar until glossy and well combined.
Whisk in the eggs one at a time, then whisk in vanilla. Continue whisking for 1 minute until the batter looks thick and shiny.
In a separate bowl, whisk together flour, cocoa powder, espresso powder, and fine sea salt. Fold the dry ingredients into the wet mixture just until no dry streaks remain.
Fold in chopped toasted hazelnuts and dark chocolate chips. Spread the batter evenly into the prepared pan.
Bake for 25 to 30 minutes, until the edges are set and a toothpick inserted near the center comes out with moist crumbs but no wet batter.
Immediately sprinkle with flaky sea salt, if using. Cool completely in the pan on a rack before lifting out and slicing into 16 squares.
Notes
For extra crackly tops, whisk the batter vigorously after adding the eggs. Do not overbake—remove when the center still has a slight wobble and the tester shows moist crumbs. Brownies keep well, covered, at room temperature for up to 3 days or can be chilled for cleaner slices.