1. In a small bowl, combine salt, cayenne, white pepper, black pepper, basil, and thyme; set spice mixture aside. In a 4-qt. heavy-bottomed pot, heat oil over high heat until it just begins to smoke. Sprinkle in flour, whisking constantly, and cook for 30 seconds. Reduce heat to medium-low and cook, whisking constantly, until roux is the color of dark chocolate, about 30 minutes. Add onions and cook, stirring constantly with a wooden spoon, until onions soften, about 5 more minutes. Remove pot from heat and stir in 1 tbsp. reserved spice mixture, along with celery and bell peppers. Continue stirring until roux has cooled and darkened slightly, about 5 minutes; set aside.

2. In a 2-qt. saucepan over medium-high heat, bring 2 cups broth to a boil. Gradually add roux and whisk until incorporated. Reduce heat to low and cook for 2 minutes more. Remove pan from heat; set broth mixture aside.

3. In a 4-qt. saucepan, melt 8 tbsp. butter over medium-high heat. Stir in crawfish tails and scallions and cook, about 1 minute. Add remaining spice mixture and reserved broth mixture, along with remaining broth and butter, and stir the pan to combine until glossy. Remove pan from heat and serve étouffée with rice.