While the water is coming up to a boil or macaroni is cooking, you can start to heat your cast iron skillet over medium heat and melt the butter.
½ cup Salted Butter
Whisk in the flour until smooth (about 3-4 minutes) and then whisk in the mustard powder.
4 tablespoon Flour, 1 tablespoon Mustard Powder
Whisk in the half and half and milk and bring to gentle boil. Turn off the heat and stir in the cream cheese until smooth.
1 cup Half and Half, 1 cup Whole Milk, 4 oz Cream Cheese
Then add your shredded cheeses, a little at a time in small handfuls, whisking until smooth.
2 Cups Cheddar Cheese, 2 Cups Gouda, ½ Cup Parmesan