Cut unsalted butter and dark chocolate into small pieces and pour into a medium bowl. Fill a small saucepan about a quarter full with hot water, then sit the bowl on top so it rests on the rim of the pan, not touching the water. Put over a low heat until the butter and chocolate have melted, stirring occasionally to mix them. Now remove the bowl from the pan.

Set your gas oven to 360 Fahrenheit

Using a shallow 20cm square tin, cut out a square of non-stick baking parchment to line the base. Now tip plain flour and cocoa powder into a sieve held over a separate medium bowl, and tap and shake the sieve so they run through together and you get rid of any lumps.

Chop the white chocolate and milk chocolate into chunks on a board so you end up with rough squares

Break 3 large eggs into a large bowl and pour the sugar in. With an electric mixer on maximum speed, whisk the eggs and sugar until they look thick and creamy, like a milk shake. This can take 3-8 minutes. You’ll know it’s ready when the mixture becomes really pale and about double its original volume.

Pour the cooled chocolate mixture over the eggy mousse, then gently fold together with a rubber spatula. Plunge the spatula in at one side, take it underneath and bring it up the opposite side and in again at the middle. Continue going under and over in a figure of eight, moving the bowl round after each folding so you can get at it from all sides, until the two mixtures are one and the colour is a mottled dark brown. The idea is to marry them without knocking out the air, so be as gentle and slow as you like – you don’t want to undo all the work you did in step 5.

Hold the sieve over the bowl of eggy chocolate mixture and resift the cocoa and flour mixture, shaking the sieve from side to side, to cover the top evenly. Gently fold in this powder using the same figure of eight action as before. Stop just before you feel you should, as you don’t want to overdo this mixing. Finally, stir in the white and milk chocolate chunks until they’re dotted throughout.

Pour the mixture into the prepared tin, scraping every bit out of the bowl with the spatula. Gently ease the mixture into the corners of the tin and paddle the spatula from side to side across the top to level it. Put in the oven and set your timer for 50 minutes. When the buzzer goes, open the oven, pull the shelf out a bit and gently shake the tin. If the brownie wobbles in the middle, it’s not quite done, so slide it back in and bake for another 5 minutes until the top has a shiny, papery crust and the sides are just beginning to come away from the tin. Take out of the oven.