Pre heat the oven to 160c/Gas 3.
Line a 20cm square cake tin with baking parchment.
Roughly chop the butter into cubes and place in a heatproof bowl with 185g of the dark chocolate. Melt over a pan of boiling water. Keep stirring and once melted remove from the heat and allow to cool.
Break the eggs into a large mixing bowl, add the caster sugar and whisk briskly using an electric hand mixer. Whisk until pale and creamy and the mixture is thick and frothy.
Pour the cooled chocolate mixture over the whisked egg and sugar and gently fold together with a rubber spatula. Fold lightly in order to prevent knocking the air out of the whisked mixture.
Now sift the cocoa and flour into the egg and chocolate, again gently folding in using a figure of eight action.
Finally, stir in half the pecans until they’re dotted all of the way through.
Pour the mixture into the prepared tin, gently easing into the corners of the tin.
Decorate the top with the remaining pecan halves.
Cook in the centre of the oven for 25 minutes. It will still be gooey in the middle but should not wobble in the tin.
Leave in the tin until cool then cut into about 12 pieces.