Clean prawns and reserve heads and shells. Heat two tablespoons olive oil in a small pan and fry heads. Add two cups of water and simmer for 1 hour, reducing liquid to 1/2 cup. Strain and discard shells. Put butter and liquid in a food processor, add Pernod and half the basil. Blend well. Shape butter into a tube and chill.
Heat fish stock to a simmer. Heat 1/4 cup olive oil in a separate saucepan and cook onion and garlic until soft and transparent. Add rice and stir to coat with oil. Add white wine and stir until absorbed. Add two cups hot stock to the rice and stir continuously until absorbed.
Continue adding 1 cup of stock at a time, stirring until absorbed, until the rice is almost cooked, about 20 minutes. Add corn kernels and prawns. Heat until prawns are just cooked, then add spinach and remaining basil until it wilts. Season with salt and pepper and stir in half the basil butter.