Bring milk, salt, cayenne, mustard, nutmeg, and 1 tablespoon butter to a simmer in a pot over medium heat. Stir in macaroni. Simmer, stirring occasionally, until firm yet tender to the bite, about 8 minutes. Do not undercook.
Meanwhile, melt remaining butter in a skillet over medium heat. Add bread crumbs and toast until golden brown, 3 to 5 minutes.
Stir cheeses into the cooked macaroni. Reduce heat to low and stir just until melted. Turn off heat, cover, and let sit for 3 minutes. Uncover, stir, and season to taste.
Spoon mac and cheese into warm serving bowls; sprinkle toasted breadcrumbs on top.
