Add steel-cut oats, water, milk, salt, and cinnamon to the Instant Pot. Stir to combine and make sure no oats are stuck to the bottom.
Lock the lid and set the valve to Sealing. Cook on High Pressure for 4 minutes with Keep Warm off if possible.
Let pressure release naturally for 10 minutes, then carefully quick-release any remaining pressure.
Open the lid and stir well. The oats will look loose at first and thicken as you stir.
Stir in vanilla, maple syrup, and butter if using. If the oats are thinner than you like, use Sauté on Low and cook 1 to 2 minutes, stirring constantly, until creamy.
Serve warm with berries and nuts or your favorite toppings.
Notes
For dairy-free, use all water or unsweetened almond milk. Leftovers keep up to 4 days refrigerated; reheat with a splash of milk or water, stirring until creamy. Avoid overfilling past the halfway mark when cooking grains in a pressure cooker.