Preheat your oven to 177 °C (175 °C) and line an 8x8-inch square baking pan with parchment paper, leaving an overhang on the sides for easy removal.

Melt the chopped dark chocolate and 120 ml butter together in a heatproof bowl set over simmering water or in short microwave intervals, stirring until smooth, then let cool slightly.

Whisk the granulated sugar, 60 ml light brown sugar, eggs, and vanilla extract into the melted chocolate mixture until glossy and completely incorporated.

Gently fold in the flour, cocoa powder, and fine sea salt using a spatula just until no dry streaks remain, then spread the batter evenly into the prepared pan.

Bake the brownie base for 20 to 22 minutes until set around the edges but still slightly soft in the center.

While the brownie base bakes, melt 60 ml butter, 80 ml brown sugar, golden syrup, and heavy cream in a small saucepan over medium heat, bringing the mixture to a gentle boil for 2 minutes to create a smooth caramel sauce.

Remove the caramel from heat, gently fold in the cornflakes until evenly coated, and spoon the mixture evenly across the hot brownie base.

Return the pan to the oven and bake for an additional 10 to 12 minutes until the topping is bubbly and golden, then remove, sprinkle with flaky sea salt, and cool completely before slicing.