Shell the crawfish, removing the meat. Save the shells and heads.
Combine crawfish shells and heads with 8 cups of water in a pot.
Bring to a boil and cook for 30 minutes. See notes.
Preheat oven to 375 F.
Strain crawfish heads, etc. and reserve crawfish stock.
You should end up with about 4 cups of crawfish stock. Add additional water, if necessary to make 4 cups.
Bring the 4 cups of stock to a boil with the salt.
Add macaroni and cook until al dente.
Strain, reserving cooking liquid.
Toss macaroni with olive oil.
In a pot, melt 2 tablespoons of the butter.
Add onion, bell pepper, garlic and celery.
Saute until softened.
Add flour and cook on medium high heat for about 5 minutes, to cook out the raw taste of the flour.
Add milk and heavy cream. Simmer on medium heat for 3-4 minutes, until thickened.
Turn off heat and add cheese, crawfish, blackened seasoning and chives. Stir to combine.
Add ½ cup of reserved cooking liquid. Stir to combine.
Transfer to square baking dish sprayed with non stick spray.
Sprinkle breadcrumbs on top.
Sprinkle a few tablespoons of cheese on top.
Transfer to oven and bake for 25 minutes.