Start the Traeger grill on Smoke with the lid open until the fire is established (4 to 5 minutes). Set the temperature to 225F and preheat, lid closed, for 10 to 15 minutes.
Make the sauce: Combine the half and half, cream cheese, 4 tablespoons of chilled butter, dry mustard, hot sauce, if using, and black pepper in a large (12-inch) well-seasoned cast iron skillet.
Place the skillet on the grill grate, uncovered, and smoke the mixture for 30 minutes. Remove the sauce from the grill and whisk until fairly smooth. Increase the Traeger temperature to 350F.
While the sauce smokes, bring a large pot of salted water to a boil on the stovetop. Cook the pasta 1 to 2 minutes less than the package directions call for; the pasta will continue to cook on the grill.
Drain, rinse with cold water, drain again, and return to the pot. Toss the cooled pasta with the shredded cheeses. Pour the smoked sauce over the pasta and cheese, stirring gently with a rubber spatula to coat the
Rinse out the cast iron skillet and dry thoroughly. Tip the macaroni and cheese into the clean skillet or a buttered 3-quart baking dish or casserole, smoothing the top. Sprinkle evenly with the breadcrumb mixture. Place on the grill grate.
Bake the macaroni and cheese until it is bubbling and the top is lightly browned, 30 to 40 minutes. Dust lightly with the paprika. Carefully transfer the skillet to a trivet or hot pad before serving.