In a mixing bowl, add the chickpea flour, water, and salt. Add the water slowly while whisking to form a smooth, thin, and watery batter.
Cover with a lid, a plate, or a towel, and let the butter stand for at least 3 hours at room temperature (note 2).
After resting time, if there is still a bit of foam on the surface, remove it with a spoon (removing the foam helps to create an even crust and prevents it from burning in the oven).
Preheat the oven to 480F (250C) and position the rack in the middle (note 3).
Pour the olive oil into a 12-inch cast iron skillet or a 12-inch round baking tin or oven-safe skillet and thoroughly coat the bottom and sides with a brush (do not use parchment paper - farinata needs to crisp on top and the bottom, so it does need oil).
Give another good stir to the batter, and gently pour it into the oiled skillet. The batter should be around ½ inch thick (1cm) maximum, as farinata needs to be wide and thin (so you get lots of crusty top on each slice).
If you add rosemary, sprinkle some leaves on top of the farinata.
Bake for 10 minutes, then turn on the broiler and cook for another 5 minutes until it is golden and crispy on top (it’s ok if it still feels soft in the middle).
Remove from the oven, sprinkle with freshly ground black pepper, let it cool for a few minutes, and serve while still warm (note 4).