Preheat the oven to 180 degrees Celsius or gas mark 4.
In a pan, carefully melt the butter and the dark chocolate and mix together. Put to one side to cool.
Whisk together the eggs and the sugar until light and fluffy using an electric whisk or stand mixer. Pour the chocolate mixture into the whisked eggs and sugar and fold.
Add your flour and cocoa powder and fold again. Pour the mixture into a lined 20x20cm tin.
Bake for 25-30 minutes, or until the middle no longer wobbles!
While the brownie cools, heat the caster sugar in a saucepan.
Stir occasionally to make sure the sugar cooks evenly. Once brown in colour, add the cream and butter (careful this may spit). Place back on a high heat and be patient, it will take around 10-15 minutes to form caramel. It will need to reach approx 120 degrees to form caramel.
Remove from the heat and stir in the cornflakes until covered. Pour the caramel cornflake mixture onto your brownies and leave to set. Once cool, cut the brownies into slabs.