In 14-inch paella pan, heat 2 tablespoons oil over medium heat until shimmering, about 2 minutes. Note: Oil should be hot but not smoking. Add chicken and ½ teaspoon salt and cook until chicken is just starting to turn white, about 2 minutes.
Add sliced chorizo and cook, stirring occasionally with wooden spoon, for 1 minute. Stir in 1 cup rice and cook for 1 minute. Stir in 2 tablespoons tomato paste, 1 teaspoon paprika, and pinch of saffron (if using) and cook until fragrant, about 30 seconds.
Add 3 cups chicken broth. Increase heat to high and bring mixture to a boil. Cook for 5 minutes - DO NOT STIR!
Reduce heat to low and simmer. Note: Small bubbles should break often across surface of until rice absorbs all liquid, about 12 minutes - AGAIN, DO NOT STIR!
Turn heat up to high, and cook until crust forms (see “Socarrat = Crispy Rice!,”below), 2 to 4 minutes - STILL, DO NOT STIR! Turn off heat and slide pan to cool burner.
Sprinkle ½ cup peas and scallions over top. Season with salt and pepper to taste and drizzle with extra oil. Serve with clean wooden spoon, making sure to scrape bottom of pan to get the socarrat.