Preheat and line: Set the oven to 350°F (180°C). Grease a 12x9-inch baking tin and line it with parchment, leaving an overhang on two sides.

Melt chocolate and butter: Put the 8.5 oz of dark chocolate and the butter in a large heatproof bowl set over a pan of gently simmering water (bowl not touching the water). Leave until melted, stirring occasionally.

Whisk in the sugars: Remove from the heat and whisk in the granulated and brown sugars with a balloon whisk until smooth. This also cools the mixture so the eggs won't scramble.

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

Fold in flour: Sift over the flour and sea salt and fold through until smooth and just combined. Pour into the prepared tin and level the top.

First bake: Bake 20 minutes, until the surface is set with a thin crust forming.

Throw in the chunks: Remove from the oven and scatter or throw the chocolate chunks at the half-baked surface so they break the crust and sink in. Return to the oven.

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

Cool and cut: Cool completely in the tin on a wire rack, then lift out by the parchment and cut into 16 squares. Serve warm with thick cream or at room temperature.