1
Oven Preparation. Preheat the oven to 180°C (356°F), or 160°C in a fan oven, and let it come properly up to temperature before anything goes in.
2
Melting the Butter. Melt the butter in a small pan over a low heat - small rather than wide, because a wide pan runs hot at the edges and can split the mixture. If you are making the chocolate version rather than the cocoa one, take the pan off the heat and whisk the dark chocolate in until it is completely smooth. Let it cool for a minute or two before it meets the eggs.
3
Mixing Wet Ingredients. In a large bowl, whisk the eggs, vanilla, caster sugar and brown sugar until the mixture turns noticeably paler and thickens enough to leave a trail, about two to three minutes. This is what dissolves the sugar and gives the crackly top, so do not hurry it.
4
Combining Chocolate Mixture. Pour in the melted butter and mix until it is fully incorporated, with no film of fat sitting separately on the surface.
5
Incorporating Dry Ingredients. Sift in the plain flour and cocoa powder and add the salt. Now beat it in firmly until the batter is thick and glossy, rather than folding it gently - that vigorous mix brings the butter back into emulsion and is what stops the brownies baking greasy.
6
Preparing the Baking Tin. Line a 25x20cm (8x10 inch) tin with baking parchment, letting it come up the sides so you can lift the whole slab out later. Pour the batter in and spread it genuinely level with a spatula, because a thick corner and a thin one bake at different speeds.
7
Baking Time. Bake for 15 minutes, and start looking at 12. The edges should be set and the middle still soft with a slight wobble. Push a toothpick into the centre, hold it there for two seconds and pull it out: a wet smear means gooey, moist crumbs mean fudgy, and a clean toothpick means overbaked.
8
Cooling and Serving. Leave them uncovered in the tin to cool completely before cutting. They carry on setting as they cool, which is why a tray that looked underdone coming out of the oven is right an hour later, and cutting them warm tears the middle. Chill the slab for an hour for clean squares, wiping the knife between cuts.