Cook pasta in salted water until just al dente.
Drain and set aside. Reserve 20g plus a splash in case you want to further thin the sauce later.
Warm the liquids. In a saucepan over medium heat, combine the milk, cream, and pasta water. Heat until steaming, not boiling.
Whisk in the sodium citrate until fully dissolved.
Melt the cheese. Gradually add the shredded cheddar and gouda, stirring constantly. Stir until the sauce is completely smooth and glossy. While sodium citrate helps keep the sauce silky, still avoid overheating, since too much heat can still mess with the texture.
Season. Add the mustard powder and smoked paprika. Stir, then taste and season with salt and pepper.
Add the cooked pasta to the sauce and stir to fully coat.
Garnish with cracked pepper, herbs, or toasted breadcrumbs for crunch. Serve immediately.