The socarrat, or crispy layer of rice that forms on the bottom of the pan, is considered a delicacy in paella and adds a rich, toasted flavor. To achieve the socarrat, after the rice is cooked and the peas, chorizo, and thyme are mixed in, uncover the pan and increase the heat to medium-high for a few minutes. Watch closely and listen for the rice to crackle, indicating the formation of socarrat. Be careful to avoid burning. Once you achieve the desired crispness, remove the pan from heat and continue with the recipe by adding the cooked shrimp and mussels as directed.