In a pot over medium heat, heat the milk until it just starts to bubble, but is not boiling, 3 to 4 minutes. Remove from heat.
In a separate, heavy-bottomed pot, heat the butter over medium heat until just melted. Add the flour; whisk constantly until the mixture turns light brown, about 3 minutes. Remove from heat.
Slowly pour the warm milk, about 1 cup at a time, into the butter-flour mixture, whisking constantly. It will get very thick at first, then thin as you add the full 3 cups.
Set the pot back over medium-high heat, and continue to whisk constantly. In the next 2 to 3 minutes, the sauce should come together and become silky and thick. Dip a metal spoon into the sauce. If the sauce coats the spoon and doesn't slide off like milk, you'll know it's ready. You should be able to run your finger along the spoon and have the impression remain. Add the salt. Use the sauce immediately, or store it in the fridge for a day or two. (It will thicken in the refrigerator and may need a little more milk to thin it.)
Make the Mac & Cheese
In a small bowl, mash together the garlic and butter to form a compound butter.
Cook the pasta in salted boiling water until a little less than al dente. Drain, rinse and drain the pasta again.
In a large, heavy-bottomed pot, combine the sauce, both cheeses and the garlic butter. Cook over medium heat, stir until the cheese is barely melted, about 3 minutes. Slowly stir in the cooked pasta and cook, stirring continuously, until the dish is nice and hot, 5 more minutes. Spoon into bowls and serve hot. If you like your Mac & Cheese baked, top with breadcrumbs and bake at 400F for 15 minutes.
Recipe by Sprung At Last at https://sprungatlast.com/2013/11/gilroy-garlic-mac-cheese/