1/2 C. (1 stick) unsalted butter, divided
1/4 C. all-purpose flour
3 C. whole milk
1 T. kosher salt plus more
1/2 tsp. freshly ground black pepper plus more
1 pound elbow macaroni (or other small pasta like orecchiette)
2 C. shredded cheddar, divided
2 garlic cloves, chopped
1 C. panko (Japanese breadcrumbs)
2 T. chopped flat-leaf parsley

Preheat oven to 400°. Melt 1/4 C. of the butter in a large saucepan over medium-high heat. Add flour and cook, whisking constantly, for 1 minute. Whisk in milk and 3 C. 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 1 T. salt and 1/2 tsp. pepper. Remove sauce from heat.

Toss pasta and 1 1/2 C. cheese in a 13 x 9 x 2-inch or other shallow 3-quart baking dish. Pour sauce over (pasta should be submerged; do not stir) and cover with foil. Bake until pasta is almost tender, about 20 minutes. Meanwhile, melt remaining 1/4 C. butter in a large skillet over medium heat (at this point, I sautéed about 1/4 C. chopped pancetta for a few minutes before adding the remaining ingredients). Add garlic, panko, and parsley and toss to combine. Season with salt and pepper. Remove foil from dish. Sprinkle with remaining 1/2 C. cheese, then panko mixture. Bake until pasta is tender, edges are bubbling, and top is golden brown, about 10 minutes longer. Let sit 10 minutes before serving.