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Preheat the oven to 180°C Fan/200°C/400°F/Gas 6.
Grease a baking sheet with a little butter and dust lightly with flour.
Combine the flour, cocoa, baking powder, salt, chocolate and pistachios in a bowl.
In another bowl, beat together the butter and sugar until pale and fluffy. Add the eggs and beat until well combined.
Stir in the flour mixture until it forms a stiff dough. Place the dough onto the baking sheet and press into a 30cm x 10cm rectangle shape. Bake in the preheated oven for 35 minutes. Remove from the oven and leave to cool on a wire rack to room temperature.
Slice into thick fingers and serve with coffee or gelato.