Combine milk and water in a medium saucepan; bring to a gentle simmer over medium.
Stir in oats; reduce heat to medium-low and cook 5 minutes, stirring occasionally.
Add cocoa, sugar, maple syrup, and salt; whisk to combine without cocoa lumps.
Cook another 3-5 minutes, stirring frequently, to desired thickness.
Off heat, stir in vanilla and chips, which melt slightly for extra creaminess.
Rest 2 minutes to thicken.
Divide into bowls; top each generously with marshmallows and optional whipped cream and chocolate shavings.
Serve immediately while hot; thin with a splash of warm milk if too thick.