Preheat the oven

• Preheat the oven to 180 °C (350 °F).
• Line an 8x8 pan with parchment paper.

Melt the butter and whisk the sugars

• Melt the butter in a pan over low heat, then remove from the heat and let it cool for a minute or two so it is warm, not hot.
• Whisk in the oil, granulated sugar and brown sugar for a full minute until glossy.

Whisk in the eggs

• Add the eggs, egg yolk and vanilla.
• Whisk for another full minute until pale and slightly thick — this gives the crackly top.

Fold in the dry ingredients

• Sift in the flour, cocoa powder, espresso powder and salt.
• Gently fold until just combined, without overmixing.

Fold in the chocolate and Oreos

• Fold the chopped dark chocolate and some of the Oreos into the batter.

Layer the batter and Oreos

• Pour half the batter into the lined pan and smooth it out.
• Add a layer of Oreos, then spread the remaining batter evenly on top.
• Sprinkle the remaining chopped Oreos over the surface.

Bake

• Bake until the edges are set and puffed and the center looks soft but not liquid.

Cool before slicing

• Remove from the oven and let cool so the brownies fully set before slicing.