In a small bowl add the raisins and cover with warm water. Set aside and let them rehydrate for about 10 minutes.
Sift the chestnut flour in a large bowl, then slowly add the water while mixing with a whisk until you get a pancake batter consistency.
Add a pinch of salt, then transfer the batter in a well oiled baking pan. It should not be thicker than 1.5 cm (½ inch). Sprinkle on top the pine nuts, the drained (and squeezed) raisins and the rosemary needles.
As a finishing touch, drizzle on top with a little bit of olive oil.
Bake in pre-heated over at 180°C (350°F) for about 25-30 minutes or until the cake is firm and the surface is dry and cracked.
Let the Castagnaccio cool down and serve tepid or room temperature.
The Castagnaccio can be stored outside of the refrigerator for about 2-3 days.