Bring milk or milk-water mixture to a gentle boil in a medium saucepan over medium-high, stirring occasionally to prevent scorching.
Reduce heat to low; slowly whisk cereal in a steady stream, stirring constantly to prevent lumps.
Cook 2-3 minutes, stirring frequently, to desired thickness.
Off heat, stir in sugar, cinnamon, vanilla, salt, and butter until combined and butter melts.
Stand 1-2 minutes to thicken slightly; it thickens further as it cools.
Divide among four bowls; add favorite toppings such as berries, bananas, nuts, extra cinnamon, or maple syrup.
Serve immediately while warm and creamy.