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1
Heat the oven to 180 °C. Combine the flour, cocoa powder, sugar, and baking powder in a bowl. Add the eggs and bring the mixture together into a firm, cohesive dough — it will be dense. Work in the whole peeled hazelnuts. No fat is added at any stage; its presence prevents complete drying and the biscuit will soften within hours. The dough is ready when it holds its shape without sticking to the hands.
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2
Divide the dough and shape into logs roughly 4 cm wide and 25 cm long on a lined baking sheet. Bake at 180 °C for 20 minutes. The logs are ready when they have puffed slightly and feel firm and set to a light press — they should yield no softness at the centre.
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3
Remove the logs from the oven and slice them immediately, while hot, on a 45° diagonal into pieces 1.5 cm thick. This is where the dish lives or dies: the cocoa dough hardens rapidly on cooling and delayed slicing causes the logs to crack unevenly along the hazelnuts rather than cleanly through them. Do not wait.
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4
Reduce the oven to 150 °C. Lay the sliced biscotti cut-side up on the baking sheet and return to the oven for 18–20 minutes. The denser cocoa dough requires longer than plain cantucci. Do not rely on surface appearance alone — chocolate dough can present a dry colour when still moist inside. The biscotti are done when they are completely rigid and produce a hollow, dry sound when tapped on the underside.
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5
Transfer to a rack and allow to cool completely in open air. The biscotti are fully cured when they snap cleanly with no give whatsoever. Stored in an airtight container they will keep for four to six weeks; the double bake is itself the preservation mechanism.