In a bowl, whisk together the chickpea flour and water until smooth. Add: 2 tablespoons olive oil, salt, black pepper, and rosemary (if using). Let the batter rest for 30 minutes to 2 hours. This hydrates the flour and produces a more tender texture.
Place your cast iron skillet in the oven and heat to 475–500°F for at least 15 minutes. The pan should be very hot.
Carefully remove the skillet and swirl in the remaining tablespoon of olive oil. Pour in the batter and quickly tilt the pan to distribute it evenly. The socca should be about 1/4 inch thick. Bake for 10-12 minutes, then switch to the broiler for 1-3 minutes until the top develops dark golden and lightly blistered spots.
Sprinkle with flaky sea salt and a little more black pepper. Cut into wedges and serve immediately.