For Béchamel (mac sauce)

Heat the milk in a pot over medium heat until it just starts to bubble, but is not boiling, 3-4 minutes. Remove from heat

Heat the butter over medium heat in a separate, heavy-bottomed pot. When the butter has just melted, add the flour and 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 when you first add the milk, and thinner as you slowly pour in the entire 3 cups. This is normal.

Once all the milk has been added, set the pot back over medium-high heat, and continue to whisk constantly. In the next 2-3 minutes the sauce should come together and become silky and thick. If the sauce coats the back of a spoon and doesn't slide off like milk, you'll know it's ready.

Use immediately or store in the fridge for a day or two (it may need a little milk to thin back out when it comes time to melt the cheese in this case.)

For Mac & Cheese

Cook the pasta in salted boiling water until a little less than al dente. Drain, rinse the pasta with cold water, and drain again.

Add the sauce and both cheeses to a large, heavy-bottomed pot and cook over medium heat. Stir until the cheese is barely melted, about 3 minutes. Slowly add the cooked pasta, stir, and continue cooking while stirring continuously until the pasta is hot and steaming, another 5 minutes.