1) Preheat oven to 200 degrees. Melt the butter in a medium saucepan set over medium heat. Sprinkle in the flour, stirring with a wooden spoon to incorporate. Continue stirring for 2 to 3 minutes until the floury taste cooks out. Slowly stir in the warm milk until a sauce forms. Add the dry mustard, paprika, and salt. Stir the Velveeta into the béchamel sauce a little bit at a time, letting the cubes melt before adding more.