Preheat the oven to 230 degrees.
Mix the chestnut flour with salt, oil and water until you have a velvety dough. Then add the pine nuts, rosemary and half of the orange zest.
Pour the dough into an oil-brushed (or parchment paper-lined) baking pan and swirl the remaining zest over the top. Make sure that the height of the cake does not exceed 2 cm!
Bake the castagnaccio for about 30-40 minutes, it is ready when the surface of the cake is cracked.
Serve the Castagniaccio lukewarm!