Brush 1 Tbs. of the butter over the bottom of the pressure cooker.
In a blender, blend the milk, cream, cream cheese, remaining 1 Tbs. butter, garlic, dry mustard, salt, pepper, cayenne, and nutmeg.
Add the cream-cheese mixture and the macaroni to the pressure cooker, and stir to combine. Cover and cook on high pressure for 6 minutes.
Release the pressure manually. Stir the noodles, and if they look dry, stir in more milk to taste. If the noodles are too al dente for your liking, stir in the cheddar, then cover and let the pot sit for 5 to 10 minutes; the noodles will cook a little more just sitting in the pot. Otherwise, stir in the cheese and serve immediately.