Preheat oven 375 degrees.
Rinse and dry chicken, then sprinkle liberally with salt and pepper. Put in refrigerator for up to 24 hours, or set aside while you prepare remaining ingredients if you're going to cook it right away.
You'll need a covered pot that will hold the chicken close, such as a Dutch Oven. Melt the butter over medium heat, then add the oil. Put in the chicken, breast side up and let it sizzle for about 30 second. Carefully flip the bird and cook the other side, another 30 seconds. Remove from the heat, put the chicken on a plate, and remove the fat in the pot.
Put your chicken back in the pot with the cinnamon stick, star anise, a handful chopped cilantro stems (about 1/2 cup), lemon, lemongrass, garlic and coconut milk and cook in the preheated oven for 60-90 minutes (depending on size), basting the sauce over the top of the bird every 20 minutes or so. The chicken is done when an instant-read thermometer inserted into the thigh reads 165° F.
Remove chicken and put on a plate. Pull out cinnamon stick, and any other solids you do not want to serve with the chicken. Put sauce back on the stovetop, add the spinach and heat over medium flame just until wilted.
Carve chicken and serve each piece over rice with sauce spooned over the top. Garnish with chopped scallions and cilantro leaves.