Melt butter in the pressure cooker, stir in vegetable broth, macaroni, Worcestershire sauce, dry mustard powder, and salt.
Secure and lock the lid in place. Set the pressure valve to HIGH pressure.
Begin heating on high to build the pressure. Once steam starts to come out of the operating valve reduce heat to maintain a steady stream of steam. Start cooking time of 7 minutes.
When cooking time is complete, release the pressure using the quick-release method.
Stir in cheeses, one cup of the hot milk, cayenne and nutmeg. Stir until cheese melts and becomes saucy and incorporated, adding more hot milk if needed.
Season to taste with salt and pepper. Serve immediately.