In a medium saucepan, combine milk and water. Bring to a gentle simmer over medium heat.
Stir in the oats and reduce heat to medium-low. Cook for 5 minutes, stirring occasionally.
Add the cocoa powder, brown sugar, maple syrup, and salt. Whisk well to combine and ensure no cocoa lumps remain.
Continue cooking for another 3-5 minutes, stirring frequently, until oatmeal reaches your desired thickness.
Remove from heat and stir in vanilla extract and mini chocolate chips. The chocolate chips will melt slightly, making it extra creamy and chocolatey.
Let the oatmeal rest for 2 minutes to thicken up a bit more.
Divide into bowls and top each serving with a generous handful of mini marshmallows. Add a dollop of whipped cream and chocolate shavings if you're feeling extra indulgent.
Serve immediately while hot and watch it disappear! Pro tip: if the oatmeal gets too thick, just stir in a splash of warm milk to loosen it up.