Preheat your oven to 190c fan forced. Grease and line a round cake tin or a flan dish with at least a 25cm base.
Start by combining the chestnut flour and salt in a large mixing bowl. Make a well in the middle and add in 2 1/2 cups of the water to start along with the 1/2 cup of extra virgin olive oil. Mix well with a wooden spoon or even a whisk if easier to bring to a batter. You are looking for a consistency similar to a pancake batter that runs off the back of the spoon. If you need to add more water add some from your remaining 1/2 cup, adding even more if you think you need it, as flours will vary.
Spoon out the batter into the lined baking dish or tin and smooth out with the back of a spoon. Now dress the top of the castagnaccio with the grapes, pine nuts, pistachios and rosemary. Finish by drizzling with the remaining extra virgin olive oil.
Place in the oven to cook for 1 - 2 hours, depending on your oven. I understand this is a varied cooking time but I find this cake is very dependent on your oven. Check the cake at the 45 minute mark. If it’s set in the middle then its good to go, otherwise keep it cooking and check every 2o minutes until cooked through. The cake will crack along the top once cooked and have a slight spring.
Once you are happy with the cooking time, remove the castagnaccio from the oven and allow to cool for 10 minutes. Remove carefully from the baking dish or flan tin and slice. Eat alone or ideally drizzled with honey.