In a one-gallon saucepot, melt butter over medium heat. Add flour and whisk until well incorporated. Note: If you don’t have a scale to weigh flour, add flour slowly until mixture resembles fine wet sand.
Cook slowly for two minutes and then add milk slowly while whisking vigorously to remove lumps. Cook over low heat until the liquid begins to slowly boil and thickens. Stir frequently with a whisk to prevent the sauce from burning.
Add salt, heavy cream, and turn the flame to the lowest setting. Stir well.
In the meantime, bring water to boil in a very large pot. Cook pasta for three to four minutes in salted, boiling water (it will still be very al dente). Drain pasta.
While the pasta drains, melt Velveeta and half the cheddar in sauce, bringing flame back up to medium. Add pasta to sauce/cheese mix. Stir well and transfer to a large baking dish.
Cover with remaining cheese and toasted panko. Cut butter into tiny cubes and distribute evenly over the dish. Bake at 350 degrees for 10−15 minutes before serving.