Preheat the oven to Gas 3, 170 °C, fan 150 °C.
Melt dark chocolate and butter in a heavy based saucepan, gently over a low heat.
Once melted, take pan off the heat and mix in the caster sugar - let the mixture cool a little, say 10 minutes or so.
Add the beaten eggs to the pan along with the ground almonds, flour and chopped walnuts (if you want to include them) - a little at a time, to ensure the mixture isn't lumpy!
Turn the mixture onto a greases 21cm (9inch square) baking tin.
Bake the mixture in the oven for 25-30 minutes - you want it looking slightly cracked and firm on top with a soft middle.
Leave to cool slightly and cut into 16 squares - eat hot out of the tin or leave them to cool completely in the fridge.
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