Preheat the oven to 350°F (175°C). Line an 8-inch (20 cm) square baking pan with parchment paper, allowing excess to hang over the sides for easy removal.

In a large heatproof bowl, melt the butter in the microwave in 20-second intervals. Add 1 cup (170g) of the chocolate chips and stir until smooth and fully melted. Microwave in additional short bursts if needed. Once melted, stir in ½ cup (120g) of Nutella until fully combined. Set aside to cool slightly.

In a separate large bowl, using an electric mixer, beat the eggs, egg yolks, brown sugar, and granulated sugar together on medium speed for approximately 4 to 5 minutes, or until the mixture is pale and thick.

Reduce the mixer speed to low. Add the vanilla extract, followed by the slightly cooled chocolate mixture. Mix just until combined.

Add the flour, cocoa powder, salt, and the remaining ½ cup (85g) of chocolate chips. Using a spatula or wooden spoon, gently fold the mixture together until no streaks of flour remain. Do not overmix.

Pour the batter into the prepared pan and smooth the top evenly. Drop spoonfuls of the remaining ¼ cup (60g) Nutella over the surface and use a butter knife to gently swirl it into the batter to create a marbled pattern. Sprinkle with flaky sea salt if desired.

Bake for 30 to 35 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs. Avoid overbaking to ensure a fudgy texture.

Allow the brownies to cool completely in the pan. Once cooled, use the parchment overhang to lift them out. Slice into squares and serve.