Salt a large pot of water like the ocean. Cook macaroni to al dente per package directions. Drain and set aside.
In the same pot, add cauliflower and enough fresh water to cover. Simmer until fork-tender, 7–10 minutes (frozen cooks faster). Drain well.
Add cooked cauliflower, milk, and butter to a blender. Blend until completely smooth and silky. If you don't have a blender, mash thoroughly and whisk—texture will be a bit more rustic, still great.
Pour the purée back into the pot over low heat. Add optional Dijon, garlic powder, and paprika. Taste and season with salt and pepper.
Stir in shredded cheddar by the handful, letting each addition melt before adding more. Keep heat low—no boiling—or the sauce can split.
Fold in the drained macaroni. Toss until every elbow is coated in creamy goodness. If too thick, splash in extra milk. If too thin, simmer 1–2 minutes to tighten.
Top with a little more cheese, a crack of pepper, or a pinch of paprika if you're feeling fancy.