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1
Heat the oven to 180 °C. Lightly oil a shallow baking pan — tin or earthenware — with a little of the measured Oliu di Corsica. The pan must be large enough that the batter, once poured, sits no deeper than 1.5 cm; a deeper pour will not dry through. The oil coat is the only preparation the pan requires. Done when the base glistens with a thin, even film.
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2
Sift the chestnut flour and salt into a wide bowl. Add the cold water in a slow, steady stream, whisking constantly, until the batter is thin, smooth, and fully pourable — no lumps of flour remaining. Work patiently; chestnut flour resists hydration. Stir in 50 ml of the olive oil. The finished batter should coat the back of a spoon only lightly and run off it in a ribbon. Done when the surface of the batter shows no dry streaks and flows freely when the bowl is tilted.
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3
Fold in the drained raisins and pine nuts. Pour the batter into the prepared pan; it must settle to a depth of no more than 1.5 cm — this is where the dish lives or dies. A batter poured thicker will remain gummy at the centre regardless of oven time, and there is no correcting it once baked. Scatter the fresh rosemary leaves evenly across the surface, then drizzle the remaining 30 ml of olive oil over in a thin, even stream. Done when the oil pools visibly in small depressions across the top of the raw batter.
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4
Slide the pan into the centre of the oven and bake at 180 °C for 30–35 minutes. Do not open the oven before the 30-minute mark. Baking past 35 minutes will make the cake brittle and excessively dry; the window is narrow. Done when the surface has cracked into a dark beige-brown map of fissures with olive oil visibly pooled in each crack, and the edges have begun to pull away from the pan.
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5
Remove from the oven and leave the castagnaccio in its pan at room temperature for a minimum of four hours before cutting. Do not attempt to slice while warm — the interior will be gummy and the cut will be ragged. The resting period allows the interior texture to settle from gummy to dense and yielding. Done when a clean knife drawn across the surface meets no resistance and the slice holds its edge without crumbling.
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6
To serve, cut into portions and place each on a plate with a sliver of brocciu passu and a drizzle of chestnut honey. The aged cheese and honey transform castagnaccio from a simple cake into a composed Corsican dessert plate. Done when the aroma at the table carries chestnut sweetness, rosemary resin, and the vegetal note of the olive oil in balance — no single element dominating.