Preheat the oven to 180°C / 355°F (conventional) or 160°C / 320°F (fan-assisted). Lightly grease a 23 cm (9-inch) square baking pan and line with baking paper, allowing overhang on all sides.

Prepare a double boiler by filling a saucepan one-third full with water and bringing it to a gentle simmer. Place a heatproof bowl over the pan, ensuring the bowl does not touch the water. Add the butter and dark chocolate to the bowl and stir until fully melted and smooth. Alternatively, melt in the microwave in 30-second intervals, stirring between each interval.

Remove the bowl from heat and whisk in the sifted cocoa powder until fully incorporated. Set aside and allow the mixture to cool to room temperature, approximately 10 minutes.

In a separate large mixing bowl, combine the eggs, caster sugar, brown sugar, vanilla extract, and salt. Using an electric mixer, whisk for 2–3 minutes until the mixture is pale, thick, and doubled in volume.

Gradually add the cooled chocolate mixture to the egg mixture in thirds, whisking thoroughly after each addition.

Add the sifted flour and gently fold with a spatula until just combined and no dry streaks remain.

Pour the batter into the prepared baking pan and smooth the surface evenly. Bake for 20–22 minutes, or until a toothpick inserted into the center comes out with moist crumbs attached.

Remove from the oven and allow the brownies to cool completely at room temperature before adding the topping.

Place the granulated sugar into a large, heavy-bottomed saucepan over low to medium heat. Stir gently as the sugar begins to clump and melt, eventually forming a smooth amber-colored caramel.

Once the caramel reaches a light golden color, remove the pan from heat. Slowly pour in the heated cream while stirring carefully, as the mixture will bubble and release steam.

Add the cubed butter and stir until fully melted and the caramel is smooth and cohesive.

Gently fold the cornflakes into the caramel mixture until evenly coated.

Immediately spread the caramel crunch mixture evenly over the cooled brownie base.

Allow the brownies to set at room temperature for approximately 30 minutes, then refrigerate for at least 4 hours, or preferably overnight, until fully firm.

To serve, remove from the pan using the parchment overhang. Slice into 4 × 4 squares using a sharp knife, wiping the blade clean between cuts for neat slices.