Preheat the oven to 180C/160C Fan/Gas 4. Butter and line a 20-23cm square cake tin (or brownie pan) with baking parchment, allowing the paper to come up over the edges (this will help you to remove the brownies).
Put half the chocolate in a bowl set over a pan of simmering water (make sure the bottom of the bowl doesn’t touch the water). Stir to help it melt then remove and allow to cool a little. Chop the rest of the chocolate into chips.
Beat the butter and sugar in an electric mixer until light and fluffy, making sure to scrape the mixture from the side of the bowl – all of it must be thoroughly mixed.
Add the vanilla then the eggs, a little at a time, beating really well after each addition, until you have a really pale, fluffy, glossy mixture.
Beat in the melted chocolate then, using a metal spoon, fold in the flour and cocoa with a pinch of salt followed by the nuts and chocolate chips. Scrape the mixture into the prepared tin.
Bake for 30 minutes until just firm to the touch (the skewer test doesn’t work as the centre will still be a bit gooey).
Leave the brownies in the tin until they’ve cooled down a bit – they will continue to cook and 'set’ – then carefully lift the paper out with the brownies on it.
Sift on a little cocoa powder (or leave them bare) and cut into squares.