Preheat and line: Set the oven to 180C (160C fan / 350F / Gas 4). Grease and line a 30x23cm (12x9 inch) cake tin with baking parchment.

Melt chocolate and butter: Put the 245g of dark chocolate and butter into a large heatproof bowl set over a pan of gently simmering water. Don't let the bowl touch the water underneath. Leave until melted, stirring occasionally.

Mix in the sugars: Remove from the heat and whisk in the caster sugar and light brown sugar using a balloon whisk. This cools down the melted chocolate so you don't scramble the eggs in the next step.

Add eggs and vanilla: Add the beaten eggs and vanilla extract, and mix in with a wooden spoon until combined. Don't overwork it.

Fold in flour: Sift over the plain flour and sea salt, then stir until smooth and just combined. Pour into the prepared tin and smooth the top level.

First bake: Bake for 20 minutes, then remove from the oven.

Throw in the chunks: Vigorously throw the 100g of dark chocolate chunks at the half-baked brownie so they break the surface and sink in. Return to the oven.

Finish baking: Bake for another 15 to 20 minutes until cracked around the edges and still soft in the middle. Rotate the tin halfway through. The brownie will firm up as it cools.

Cool and cut: Transfer to a wire rack and leave to cool in the tin, then cut into squares. Serve warm with thick cream or cool completely for a chewier texture.