Melt the butter in a large pot over medium heat. Add the breadcrumbs and cook, stirring often, until they’re golden and crisp, 3 to 5 minutes. Transfer to a bowl; wipe out the pot.
Put 3 cups milk and the pasta in the pot and sprinkle with salt. Bring the mixture to a boil, stirring often, then adjust the heat so it bubbles steadily. Cook, stirring constantly, for 5 minutes, then start tasting. You want the pasta to be tender but still with a tiny bit of crunch; it could take as long as 15 minutes. Meanwhile, the mixture should be neither soupy nor dry; add 1/4 cup milk if necessary.
Stir in the cream cheese, cheddar, and enough of the remaining milk to make a thick sauce. Stir until the cheese is melted and fully incorporated. Taste and adjust the seasoning. Top with the breadcrumbs and serve hot.