Step 1: Melt chocolate & butter
Add the butter and dark chocolate to a heatproof bowl. Set it over a small pan of simmering water (don’t let the bowl touch the water). Stir occasionally until melted and smooth, then remove from heat and let cool to room temperature. Alternatively, microwave in 30-second bursts.

Step 2: Prep the oven & tin
Preheat the oven to 180°C (160°C fan) / gas mark 4. Line a 20cm square baking tin with foil or baking parchment.

Step 3: Sift dry ingredients
Sift the flour and cocoa powder into a bowl to remove any lumps.

Step 4: Beat eggs & sugar
In a large bowl, whisk the eggs and sugar with an electric mixer on high for 3–8 minutes, until pale, thick, and doubled in volume. You should be able to drizzle a ribbon from the beaters that sits on top briefly before sinking.

Step 5: Combine chocolate & eggs
Pour the cooled chocolate mixture over the egg mixture. Gently fold together using a figure-of-eight motion with a spatula. Be careful not to knock out the air.

Step 6: Add flour & cocoa
Sift the flour and cocoa mixture again over the batter. Fold in gently, just until the mix looks thick and fudgy—don’t overmix.

Step 7: Stir in chocolate chunks
Fold in the chopped white and milk chocolate, distributing evenly.

Step 8: Bake
Pour the batter into the tin, spread evenly, and bake for 25 minutes. Gently shake the tin—if it wobbles in the middle, bake for another 5 minutes. It’s done when the top looks shiny and slightly cracked, and the edges are pulling away from the tin.

Step 9: Cool & cut
Let the brownies cool completely in the tin. Once cool, lift out and cut into squares or triangles.

Step 10: Store or freeze
Store in an airtight container for up to 2 weeks, or freeze for up to a month.