In a large pot, bring 4 quarts of salted water to a rolling boil over high heat. Add the elbow macaroni and cook for 3 to 5 minutes past the package's al dente time (about 10 minutes total), stirring occasionally, until the pasta is very tender — this extra time is what gives KFC-style mac its soft, restaurant-style bite. Drain well and return the pasta to the pot.

Set the same pot over low heat and add the heavy cream, mustard powder, salt, Velveeta, and shredded cheddar. Stir constantly for 3 to 4 minutes, keeping the heat low the entire time, until the cheeses have melted completely into a smooth, glossy sauce that clings to the noodles. Do not let the sauce boil — high heat is the most common reason a cheese sauce turns grainy or breaks.

Remove the pot from the heat and give the mac and cheese one final stir to make sure the sauce is evenly distributed. Serve immediately while it's hot and the sauce is at its creamiest; it will continue to thicken as it sits, so stir in a splash of extra heavy cream if it's not loose enough for your taste.