Add all ingredients to a stovetop pressure cooker.
Close the lid. If the pressure cooker has a detachable pressure regulator, remove it. Then, reattach it later when steam flows vigorously from the vent (this purges air from the pressure cooker and replaces it with steam for more effective pressure-cooking.).
Turn the burner to high. When the cooker comes to full pressure (15 psi), immediately turn the burner down to the lowest burner setting that will maintain 15 psi.
Pressure-cook for 5 minutes using a low burner setting (a medium burner setting may cause scorching). Turn off the burner.
Let the pressure cooker sit on the burner for at least 15 minutes to finish cooking and slowly depressurize.
Open the pressure cooker (if it's still pressurized, I use a knife to lift the pressure regulator, which lets out any remaining pressure).
Whisk in butter if desired and break up any congealed polenta at the bottom.
