Cut butter into smallish cubes and tip into a medium bowl. Break milk chocolate into small pieces and drop into the 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 and allow to cool. 

While you wait for the chocolate to cool, position a shelf in the middle of your oven and turn the oven on to fan 160 degrees then line a 20cm square tin with non-stick baking parchment to line the base. 

Now tip flour and cocoa powder into a sieve held over a medium bowl, and tap and shake the sieve so they run through together and you get rid of any lumps. With a large sharp knife,  chop the dark chocolate into chunks on a board. 

Break eggs into a large bowl and add the caster sugar. With an electric mixer on maximum speed, whisk the eggs and sugar until they look thick and creamy. Pour the cooled chocolate mixture over the eggy mousse, then gently fold together with a rubber spatula. 

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, stopping just before you feel you should, as you don’t want to overdo this mixing. Finally, stir in the dark chocolate chunks and walnuts until they’re dotted throughout. 

Pour the mixture into the prepared tin, gently easing the mixture into the corners of the tin. Put in the oven and set your timer for 25 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.

Leave the whole thing in the tin until completely cold, then remove and cut into triangles to share (or eat it all before your husband gets home from work – what? I am a breast feeding Mum).