Prep the cheese. Shred the cheddar and Jack on the large holes of a grater. Freshly grated melts smoother than pre-shredded. Set aside.

Boil the pasta. Bring a large pot of salted water to a boil. Cook gluten-free pasta 1 minute shy of al dente per package directions. Reserve 1/2 cup pasta water, then drain.

Make the cornstarch slurry. In a small bowl, whisk 1 tablespoon cornstarch with 2 tablespoons cold milk until smooth. This prevents lumps.

Warm the dairy base. In a wide saucepan over medium heat, melt butter. Add remaining milk and cream. Whisk in mustard powder, garlic powder, onion powder (if using), 1/2 teaspoon salt, and a few grinds of pepper. Heat until steaming but not boiling.

Thicken the sauce. Whisk the cornstarch slurry, then pour it into the hot milk while stirring. Simmer 1–2 minutes until slightly thickened and glossy.

Add cream cheese. Whisk in cream cheese until fully melted and smooth. Keep heat on low to avoid scorching.

Melt in the cheeses. Off the heat, add cheddar and Jack a handful at a time, whisking until each addition melts before adding more. If you see any graininess, add a splash of hot pasta water and whisk until smooth.

Season and brighten. Stir in hot sauce (to taste). Taste and adjust salt and pepper. For extra umami, whisk in Parmesan.

Combine with pasta. Add the drained pasta to the sauce. Toss over low heat for 1–2 minutes to finish cooking and help the sauce cling. Use splashes of reserved pasta water as needed to keep it silky.

Serve immediately. The sauce will be ultra creamy right off the stove. Garnish with cracked pepper or chives if you like.