Preheat your oven to 180°C (355°F) and grease a 9-inch square baking tin, lining it with baking paper.

In a heatproof bowl, combine unsalted butter and dark cooking chocolate over simmering water, stirring until smooth (5-7 minutes).

Whisk in the Dutch process cocoa powder until smooth and let it cool for about 5 minutes.

In a separate bowl, whisk large eggs, caster sugar, brown sugar, vanilla extract, and a pinch of salt until pale and fluffy (3-5 minutes).

Pour the cooled chocolate mixture into the egg and sugar mix and combine gently.

Sift in all-purpose flour and fold it in until just combined, ensuring no dry flour remains.

Pour the brownie batter into the prepared tin, smoothing the top, and bake for 20-22 minutes.

In a saucepan, melt granulated sugar over medium heat, stirring until golden brown, then add hot heavy cream and salted butter, mixing until smooth.

Fold in cornflakes until evenly coated and spread over cooled brownie base.

Allow to set at room temperature for 30 minutes, then refrigerate for at least 4 hours before slicing.