Preheat oven to 350°F (175°C). Line an 8-inch (20 cm) square baking pan with parchment paper leaving an overhang on two sides for easy removal; lightly grease the paper.

Make brownie base

Melt 115 g unsalted butter and 115 g semisweet chocolate together in a heatproof bowl set over barely simmering water (or in short bursts in the microwave, stirring every 20–30 seconds) until smooth. Remove from heat and let cool 3–5 minutes. Whisk in 150 g granulated sugar until combined. Add 2 large eggs, one at a time, whisking until glossy. Stir in 1 tsp vanilla. Sift in 85 g all-purpose flour, 15 g unsweetened cocoa powder, and 1 tsp fine salt; fold gently just until no streaks remain. Do not overmix.

Transfer base to pan

Spread the brownie batter evenly into the prepared pan using a spatula, leaving room on top for the cookie dough and Nutella swirls.

Make cookie dough

In a medium bowl, cream 70 g softened unsalted butter with 50 g packed light brown sugar and 25 g granulated sugar until light and fluffy (about 1–2 minutes). Mix in 1 large egg yolk and 1/2 tsp vanilla until combined. Stir in 90 g all-purpose flour, 1/2 tsp baking powder, and 1/4 tsp fine salt until a soft cookie dough forms. The dough should be pliable but not sticky; if too soft, chill 10 minutes.

Assemble layers

Drop small spoonfuls (about 1 tsp to 1 tbsp each) of the cookie dough across the surface of the brownie batter, spacing evenly. Warm 200 g Nutella slightly (10–15 seconds in microwave) until pourable but not hot. Dollop Nutella over the surface in several places. Use a skewer or knife to swirl Nutella and cookie dough into the brownie batter to create a marbled pattern—avoid fully mixing; you want visible swirls and pockets of Nutella and cookie dough.

Bake

Bake in the preheated oven for 22–28 minutes, or until the edges are set and a toothpick inserted into the center comes out with a few moist crumbs (not raw batter). Rotate the pan halfway through baking for even color.

Finish and cool

Remove from oven and immediately, while warm, use the parchment overhang to lift the brookie out of the pan to a wire rack. If desired, lightly sprinkle 1/4 tsp flaky sea salt evenly over the top while still warm to enhance the Nutella flavor. Allow to cool at least 1 hour before slicing so layers set (chill 20–30 minutes to speed up).

Serving & storage

Once cooled and set, cut into 9 squares using a sharp knife wiped between cuts for clean edges. Serve at room temperature. Store airtight at room temperature for up to 3 days or refrigerate up to 7 days. Rewarm briefly in microwave (8–12 seconds) before serving if desired to loosen Nutella pockets.