Heat smoker to 225 degrees. Bring a large pot of salted water to a boil. Cook the elbow macaroni one to two minutes less than what the package directions dictate. Drain and reserve in a large mixing bowl.
While the water boils and the pasta cooks, melt 1 tablespoon butter in a large skillet over medium-high heat. Add the panko crumbs and stir to coat. Continue to cook, stirring every so often, until the panko crumbs turn golden brown. Remove from heat and reserve.
Melt the remaining ½ cup butter in a large skillet over medium heat. Add garlic and cook for 30 seconds, stirring constantly. Whisk in the flour. Continue to whisk constantly for about three minutes.
Slowly pour in the heavy cream and milk, whisking constantly to create a smooth mixture. Season with spicy brown mustard, salt, black pepper and smoked paprika. Bring to a simmer, whisking frequently. Do not boil as that can cause the milk to separate and leave you with a grainy sauce.
Turn off the heat. Stir in 1 cup cheddar, 1 cup Gouda and 1 cup Gruyère until the cheese melts and becomes one cohesive mixture.
Pour the cheese sauce on top of the reserved macaroni and stir to coat.
Pour half the mac and cheese into a greased 9-inch-by-13-inch baking dish or 12-inch cast-iron skillet. Layer half the remaining cheese on top. Pour the rest of the macaroni on the cheese layer and top with the last of the cheese. Sprinkle the browned panko crumbs on top.
Place in the smoker with a water pan and close the lid. Smoke for one hour. The sides should be hot and bubbly when ready. Enjoy!