Cut 185g unsalted butter into small cubes and tip into a bowl with broken dark chocolate pieces.

Quarter fill saucepan with hot water, then sit the bowl on top without touching the water. Put over a low heat, stir occasionally until the butter and chocolate have melted then remove bowl from pan. (Alternatively, cover the bowl loosely with cling film and put in the microwave for 2 minutes on High. Leave the melted mixture to cool to room temperature.)

Preheat oven on to 180C/ 160C fan/ gas 4 and put shelf in middle of oven.

Using a shallow 20cm square tin, line base with non-stick baking parchment.

Tip plain flour and cocoa powder into a sieve held over a medium bowl and sift.

Chop white and milk into chunks.

Break free-range eggs into a large bowl and add golden caster sugar then whisk until thick and creamy. You’ll know it’s ready when the mixture is pale, about double original volume and leaves a trail on surface for a couple of seconds.

Pour the cooled chocolate mixture over the eggy mousse, then gently fold together with a rubber spatula in a figure of eight movement.

Resift the cocoa and flour mixture then gently fold into the mixture until it looks fudgy

Finally, stir in the white and milk chocolate chunks.

Pour the mixture into the prepared tin and place in oven.

After 25 minutes 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.

Leave until completely cold, then cut into 16 squares (or 32 triangles). You can dust with cocoa or icing sugar, or just leave plain.