Preheat the oven to 350°F.

Place the butter in a pan over low heat and allow it to melt. Remove it from the heat and let it cool for one or two minutes. The butter should remain very warm but should not be hot.

Add the neutral oil, granulated sugar, and packed brown sugar to the melted butter. Whisk continuously for one full minute, or until the mixture looks smooth and glossy.

Add the two whole eggs, additional egg yolk, and vanilla extract. Whisk for another full minute, until the mixture becomes pale and slightly thick. This thorough whisking helps create the brownies’ crackly top.

Sift the all-purpose flour, unsweetened cocoa powder, instant espresso powder, and salt into the wet mixture.

Gently fold the dry ingredients into the wet ingredients just until combined. Be careful not to overmix the batter.

Fold approximately half of the dark chocolate into the brownie batter, reserving the remainder for the top.

Pour half of the brownie batter into the pan and smooth it into an even layer.

Add a layer of Nutella over the batter and spread it evenly.

Spoon or pour the remaining brownie batter over the Nutella. Smooth the surface gently, taking care not to disturb the middle layer more than necessary.

Sprinkle the reserved dark chocolate over the top.

Bake for approximately 25 to 30 minutes. The edges should look set and puffed, while the center should look soft but not liquid.

Remove the brownies from the oven and allow them to cool for at least 30 to 45 minutes before slicing. The center will continue to set during this time. If the brownies are cut too early, they may look raw even though they will firm up as they rest.