Preheat oven to 375 degrees.

Cook the pasta in a large pot in accordance with the instructions on the package.

Drain the macaroni and rinse with cold water. Set aside.

Heat a skillet or saucepan on medium-high heat. Add in 1 tablespoon of butter. When melted, add in the shrimp and Old Bay seasoning and optional Cajun seasoning.

Cook for 3-4 minutes on both sides until cooked through. Add in the crab and stir. Remove from the heat and set aside. Keep the seafood in the butter sauce.

Heat a skillet or saucepan on medium heat. Add 4 tablespoons of butter and allow it to melt.

Once melted, add the flour and whisk. Add the flour in stages to prevent clumping.

This will create a roux. Continue to whisk until the four is fully combined.

Add the milk and heavy whipping cream and stir.

Add half of the shredded cheese and the 4 ounces of cream cheese stir until the cheese is melted. You don't have to wait for the all of the cream cheese to melt. It will melt while the dish bakes.

Add the onion powder, salt, and pepper to taste. Taste repeatedly and adjust seasonings as needed. This step is important because this sauce is what the mac and cheese will taste like.

Return the pasta to the large pot. Drizzle the cheese sauce over the pasta and stir until fully combined.

Add in the cooked seafood and the butter sauce the seafood was cooked in. Stir. Taste again. Add additional spices, salt, and pepper if needed.

Spray a 9 x 13 inch baking pan with cooking spray.

Begin to layer in the pasta. Start with the pasta and seafood, then add a layer of all of the shredded cheese.

Add another layer of pasta, and then another layer of cheese. If your dish is piled high (mine usually is), place it on a sheet pan while baking. This will catch any potential drippings.

Bake for 25-45 minutes until the cheese has melted and is bubbly. I like for the crust on top to turn a deep golden brown, which takes more time. Use your judgment based on what works for you.

Cool before serving.