Bring a large pot of water to a rolling boil over high heat, then add 1 tablespoon of salt. The water should taste like the ocean, which will properly season the pasta from the inside out.
Add the elbow macaroni to the boiling water and cook according to package directions until al dente, usually 7-8 minutes. You want the pasta to have a slight bite to it since it will continue to absorb sauce after draining. Drain the pasta in a colander and set aside, but do not rinse as the starch helps the sauce cling to the pasta.
While the pasta cooks, melt the butter in a medium saucepan over medium heat until it's completely liquefied and just beginning to foam, about 2 minutes. The butter should sizzle gently but not brown.
Sprinkle the flour over the melted butter and whisk continuously for 2-3 minutes to create a roux. The mixture should become smooth and paste-like, with a light golden color and a slightly nutty aroma. This cooking step removes the raw flour taste.
Gradually pour in the warmed milk and heavy cream while whisking constantly to prevent lumps from forming. Continue whisking for 4-5 minutes until the sauce thickens enough to coat the back of a spoon. The sauce should be smooth and creamy with no flour lumps visible.
Reduce the heat to low and add the shredded white cheddar and Vermont white cheddar cheese in three batches, stirring after each addition until completely melted. The cheese should melt smoothly into the sauce, creating a glossy, velvety texture. Be patient and don't rush this step or the cheese may become grainy.
Stir in the Dijon mustard, garlic powder, onion powder, salt, and white pepper until fully incorporated. Taste the sauce and adjust seasoning as needed - it should be rich, creamy, and have a pleasant sharp cheddar flavor with a subtle tang from the mustard.
Add the drained pasta to the cheese sauce and stir gently but thoroughly until every piece of macaroni is evenly coated with the creamy sauce. The pasta should be luxuriously creamy and the sauce should cling to each elbow. Serve immediately while hot and creamy, as the sauce will thicken as it cools.
