Preheat the oven to 180°C (160°C fan) and line a 20cm square baking tin with parchment paper. Leave an overhang on two opposite sides — this will be your handles to lift the finished bars out cleanly. The paper should feel crisp and taut against the sides of the tin.

Make the brownie base. In a large bowl, whisk the melted butter with both sugars until the mixture looks smooth and glossy, about 1 minute. The grains of sugar should feel almost dissolved when you rub a tiny bit between your fingers. Add the eggs one at a time, whisking vigorously after each until the batter is thick and falls in a ribbon from the whisk. Stir in the vanilla.

Fold in the dry ingredients. Sift the flour, cocoa powder, and salt over the wet mixture. Fold gently with a spatula until you see no more streaks of white flour. The batter should be thick, dark, and smell deeply of chocolate. Pour it into the prepared tin and spread it evenly to the corners. The surface should be smooth and level.

Bake the brownie base for 20-22 minutes. The edges should look set and slightly pulling away from the sides of the tin, but the centre should still wobble just a little when you gently shake the tin. A skewer inserted into the centre should come out with a few moist crumbs clinging to it — not wet batter, but not clean either. Set the tin on a wire rack and let it cool completely in the tin. You want it to be cool to the touch before adding the next layer.

Make the Oreo truffle layer while the brownie cools. Place the Oreo biscuits in a sealed plastic bag and crush them with a rolling pin until you have a mix of fine crumbs and small, pea-sized chunks. You want texture, not dust. In a medium bowl, beat the softened cream cheese with the icing sugar until smooth, pale, and fluffy — about 2 minutes with a hand mixer. Fold in the crushed Oreos until evenly distributed. The mixture should be thick, slightly sticky, and studded with dark biscuit pieces.

Assemble the bars. Once the brownie base is completely cool, dollop the Oreo truffle mixture over the top. Use an offset spatula or the back of a spoon to spread it evenly, pressing gently so it adheres to the brownie. The layer should be about 1cm thick and completely cover the brownie surface. Place the tin in the fridge for at least 20 minutes to firm up.

Make the chocolate topping. In a small heatproof bowl over a pan of barely simmering water (or in the microwave in 20-second bursts), melt the chopped dark chocolate with the double cream. Stir until the mixture is completely smooth, shiny, and runs off the spoon in a thick ribbon. Let it cool for 2 minutes — it should feel warm but not hot to the touch.

Finish and chill. Pour the melted chocolate over the chilled truffle layer. Tilt the tin gently so the chocolate runs to the edges and covers the surface evenly. Return the tin to the fridge for at least 30 minutes, or until the topping is firm and no longer sticky when you touch it gently. The surface should look set and have a slight sheen.

Slice and serve. Lift the bars out of the tin using the parchment overhang. Place on a cutting board with a sharp knife. For clean slices, warm the knife under hot water, dry it, and cut with one swift motion. Wipe the blade clean between each cut. The bars should reveal three distinct layers: a fudgy brownie base, a creamy Oreo truffle centre, and a glossy chocolate cap.