Step 1: Preheat the Oven and Prepare the Baking Dish - Preheat your oven to 400 degrees F. Lightly grease a large baking dish so the cheesy pasta does not stick around the edges. I like to get this done first because once the sauce starts coming together, everything moves fairly quickly. A large casserole dish gives the macaroni enough room to spread into an even layer, which also means you get more of that cheesy, crunchy topping in every serving.
Step 2: Cook the Pasta Until Just Tender - Bring a large pot of water to a boil and cook the elbow pasta according to the package directions. I like to stop right around the point where the noodles are tender but still have a little firmness because they are going into the oven afterward. Drain the pasta well, return it to the pot, and set it aside while you make the cheese sauce. Leaving the pasta in the large pot also gives you plenty of room to stir everything together later.
Step 3: Melt the Butter - Place a medium pot over medium heat and add the butter. Let it melt completely, but keep an eye on the heat so it does not begin to brown. You are looking for a smooth base for the sauce rather than a toasted butter flavor. Once the butter has melted, you are ready to add the flour.
Step 4: Cook the Flour and Add the Milk - Whisk the flour into the melted butter and cook it for 1 to 2 minutes. This short cooking time helps remove the raw flour taste while building the base that will thicken your cheese sauce. Slowly pour in the milk while whisking continuously. Adding it gradually is worth the extra few seconds because it helps keep the sauce smooth and prevents stubborn flour lumps from forming.
Step 5: Thicken the Sauce - Continue heating the milk mixture over medium heat for about 5 minutes, stirring frequently. You should notice it beginning to thicken enough to lightly coat your spoon. It does not need to become extremely thick at this stage because the cheese will add more body and the sauce will continue settling as the casserole bakes.
Step 6: Add the Cheese, Crab, and Seasonings - Stir 1 cup of grated cheese into the warm sauce and keep stirring until the cheese is completely melted and the mixture looks smooth. Add the salt, black pepper, smoked paprika, and drained crab meat. Stir gently so you distribute the crab without breaking every piece apart. I like finding little bites of crab throughout the finished dish, so there is no need to stir aggressively.
Step 7: Combine the Pasta and Cheese Sauce - Pour the crab cheese sauce over the cooked pasta in the large pot. Use a silicone spoon to stir until the noodles are evenly coated and the crab is distributed throughout. Take a second to scrape along the bottom of the pot so no plain noodles are hiding underneath. Every scoop should have plenty of creamy sauce.
Step 8: Assemble the Crab Mac and Cheese - Transfer the pasta mixture to your prepared baking dish and spread it evenly from one side to the other. Sprinkle the remaining 1 1/2 cups of grated cheese over the pasta, then scatter the panko bread crumbs evenly across the top. Try to cover the whole surface rather than piling everything in the center because those crisp edges are some of the best bites in the pan.
Step 9: Bake Until Hot and Golden - Bake the Crab Mac and Cheese for about 15 minutes. The cheese on top should be melted, the edges should be bubbling, and the bread crumbs should have a light golden color. Let the baking dish sit for a few minutes before serving. That short rest gives the sauce a chance to settle around the noodles and makes it easier to spoon onto plates without everything running together.