Preheat oven to 400 degrees F. Spray a baking dish with nonstick spray.
Add the pasta and about 1 cup of cheddar cheese to the prepared baking dish, set aside.
Melt 4 tablespoons butter in a large saucepan over medium-high heat. Add flour, whisking constantly, for about 2 minutes. Whisk in milk and 1 1/2 cups water. Bring to a boil, reduce heat to a simmer, and cook, whisking often, until a very thin, glossy sauce forms, about 10 minutes. Stir in the mustard powder, cayenne pepper (if using), salt and pepper. Remove the sauce from the heat. Stir in the remaining 2 1/2 cups of cheddar cheese until smooth.
Pour the sauce over the pasta, stirring to coat. The pasta should be submerged in the sauce. Cover with foil and bake until pasta is almost tender, about 25 minutes.
Meanwhile, wipe your same sauce pot clean, then add remaining 4 tablespoons melted butter. Add garlic and breadcrumbs, toss to combine. Season with salt and pepper. If desired you can toast the crumbs over medium heat for 4-5 minutes until fragrant.
Remove the pasta from the oven and remove the foil from dish. Sprinkle with remaining 1/2 cup havarti cheese, then the breadcrumb mixture. Bake until pasta is tender, edges are bubbling, and top is golden brown, about 10-20 minutes longer. Let sit 10 minutes before serving. Garnish with