Preheat the oven to Gas Mark 4, 180℃, fan 160℃. Line a rectangular tin (27cm x 17.5cm x 3.5cm) with nonstick baking paper. Break 175g (6oz) of the chocolate into pieces and put into a glass bowl over a pan of simmering water along with the butter. When melted, take the bowl off the pan and put aside.
Chop the rest of the chocolate into small chunks and set aside.
Whisk the eggs and sugar together in a large bowl using an electric hand mixer, until thick, pale, fluffy and almost doubled in volume. Pour in the cooled melted chocolate, and gently fold, before sifting in the flour and cocoa and gently folding again. Lastly stir in the chopped chocolate.
Pour the mixture into the lined tin and lightly spread it into all the corners. Bake for about 20 minutes, or until the middle is just set and the top is crusty. Remove from the oven and leave to cool in the tray. Finish off by sprinkling over some cocoa powder.
Freezing and defrosting guidelines
Bake according to the recipe but stop before adding decoration. Allow to cool completely. Wrap well in clingfilm and tin foil (to prevent freezer burn) and freeze for up to 3 months.
Loosen the tin foil and clingfilm but leave to defrost in packaging at room temperature for several hours before serving. If you wish to serve the baked goods warm, place them on a baking sheet for 10 minutes in a moderate oven.
If you wish to add icing or decoration, add it after the baked goods have fully defrosted and are at room temperature.
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