Start by boiling a large pot of water. Add a generous amount of salt and cook the macaroni until just al dente, according to the package directions. Drain the pasta and rinse with cold water to stop the cooking process. Set aside.
In a small saucepan, combine the milk and cream. Heat on low until warmed through. It’s crucial that the dairy is not cold when you mix it into the sauce.
In a large skillet, melt the butter over medium-low heat. Add the flour and whisk to combine. Continue whisking for 4-5 minutes until the mixture is golden and smooth, cooking out the raw flour taste.
Gradually add the warmed milk and cream to the roux, 1/2 cup at a time, whisking continuously to thicken the sauce. Keep whisking until the sauce is thickened, but do not let it simmer rapidly.
Once the sauce begins to thicken, turn off the heat. Combine the shredded cheeses, reserving 1 cup. Gradually add the remaining cheese, 1/2 cup at a time, whisking until melted and smooth. If needed, return the pan to low heat to melt the cheese completely. Taste and adjust the seasoning as necessary.
Preheat the smoker to 250°F. Place the pan, uncovered, in the smoker. Smoke for 45-60 minutes without stirring.
Season the ground beef with salt and pepper, and form into six equal patties. Preheat your grill to medium-high heat. Cook the burgers for about 4-5 minutes per side, or until they reach your desired level of doneness.
Toast the burger buns on the grill until golden brown. Place a grilled burger patty on the bottom bun, top with a generous scoop of smoked mac and cheese, and add lettuce, tomato, onion slices, and BBQ sauce if desired. Cap with the top bun and serve immediately.