Pre-heat the oven to 180C, and line a 28cm x 18cm baking tin with baking parchment.
Set a bowl over, but not touching, a pan of simmering water, and add 200g of the chocolate, broken into pieces. Allow to melt, stirring occasionally, and then remove from the heat immediately.
Meanwhile, beat the butter and sugar together until light and fluffy, and break the rest of the chocolate into chips.
With the mixer still running, gradually add the eggs, beating well between each addition to ensure it's thoroughly incorporated before pouring in any more. Leave mixing on a high speed for five minutes until the batter has a silky sheen, and has increased in volume.
Remove the bowl from the mixer, and gently fold in the melted chocolate and chocolate chips with a metal spoon, followed by the sifted flour, baking powder, salt, cocoa powder and nuts.
Spoon the mixture into the tin, and bake for 35 minutes. Test with a skewer; it should come out sticky, but not coated with raw mixture. If it does, put it back into the oven for another 3 minutes, then test again.