The pan comes off the heat before the cheese goes in, a handful at a time, because cheese melted over a flame breaks and turns oily. Shred it from the block too: bagged shreds carry a starch that makes the sauce grainy.

Shred the Gouda, cheddar and Gruyère off the block and toss them together. Bagged shredded cheese is coated in starch to keep the shreds apart, and that starch is what turns a cheese sauce grainy.
Bring a large pot of water to a boil and add the tablespoon of salt. Cook the pasta a minute short of the package's al dente time, about 8 minutes; it finishes softening in the sauce. Scoop out the measured pasta water, then drain.
While the pasta cooks, melt two-thirds of the butter in a large saucepan or Dutch oven over medium heat. Whisk in the flour and keep whisking for about 2 minutes, until it smells nutty but hasn't taken on any color.
Pour in the warm milk a splash at a time, whisking constantly. Warm milk loosens the roux without seizing it into lumps, which cold milk will. Whisk in the cream, bring it to a gentle simmer, and cook about 7 minutes, whisking often, until the sauce coats the back of a spoon. Half-and-half in place of the cream gives a lighter sauce that tightens sooner, so keep the pasta water close.
Take the pan off the heat before the cheese goes anywhere near it; over a flame, cheese breaks and turns oily. Whisk in the dry mustard, garlic powder, smoked paprika, the salt and the pepper. Add the cheese a handful at a time, and let each one melt before the next goes in.
Fold in the drained pasta. If the sauce is too thick, loosen it with a little of the pasta water. Taste for salt and serve straight away; it thickens as it stands.
For a baked version, heat the oven to 375°F and spread the mac and cheese in a 9x13 dish. Melt the remaining butter, toss the panko in it, scatter it over the top and bake for 20 minutes, until the crumbs are golden.
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