Fudgy Brownie Layer:

Preheat and Prep:

1. First, set your oven to 170°C (350°F). If your oven doesn’t have a fan, bump it up to 190°C (375°F). Grab a 9x13-inch (23x33 cm) baking tin, spray it with a bit of oil, and line it with baking paper. Leave some extra paper hanging over the sides so you can lift the brownies out easily. Believe me, it’ll save you a lot of hassle. If the paper keeps slipping, use bulldog clips to hold it in place—works like a charm!

Melt the Sugar and Butter:

1. In a heatproof bowl, mix together the brown sugar, caster sugar, salted butter, and chopped dark chocolate. Microwave it in 30-second bursts, stirring with a whisk in between, until everything is melted and smooth. Don’t rush this part; let it all blend into a silky, delicious mix.

Whisk in the Eggs:

1. Now, add in the eggs and that extra egg yolk. Give it a good whisk until it’s all nicely combined. Now, go ahead and stir in the flour, cocoa powder, and a splash of vanilla extract. Keep stirring until the batter looks smooth and perfectly mixed, and you're good to go.

Bake:

1. Pour the batter into your prepared baking tin, smoothing out the top. Pop it in the oven and let it bake for about 25-30 minutes, or until the brownies are nicely set. Let them cool completely to room temperature before adding the next layer. You don’t want to rush this step—cooling keeps the layers from mixing.

Cornflake Layer:

Make the Toffee (Patience is Key!):

1. Pour the granulated sugar into a large saucepan and heat it over low to medium heat. Stir it gently with a wooden spoon until it starts to melt. Let it bubble and cook until the sugar turns into a rich, golden toffee and reaches about 118°C (245°F). Keep a close eye on it because it can go from perfect to burnt really quickly!

Add Cream and Butter:

1. Slowly pour in the hot heavy cream, stirring as you go, then add the butter. Keep mixing until it all comes together into a smooth caramel. Turn up the heat a bit, and let it cook for another 5 minutes or so until the caramel reaches 120°C (250°F). You’re looking for a caramel that’s thick, glossy, and just downright irresistible.

Mix in the Cornflakes and Spread:

1. Take the pan off the heat, then add the cornflakes. Gently fold them through the caramel with a wooden spoon until every piece is coated. Pour this sticky, crunchy mix over the cooled brownie layer and spread it out evenly.

Set and Slice:

1. Let the whole thing sit at room temperature for about 30 minutes, then pop it in the fridge for 2 hours to firm up. This helps everything set, so slicing is a breeze.

Perfect Slices Every Time:

1. When you’re ready to cut, grab a sharp knife and run it under hot water, then dry it off. This trick helps you get clean cuts through that sticky caramel cornflake layer. Cut into 4x5 squares, and there you have it! Serve chilled or at room temp, whichever you prefer.