Preheat oven to 350°F (175°C) and grease a 9×13-inch baking pan.
Mix the cake mix, melted butter, water, eggs, and vanilla extract in a large bowl until smooth.
Pour the batter into the prepared pan and bake for 30–35 minutes, until a toothpick inserted comes out clean.
Remove cake from oven and immediately sprinkle mini marshmallows evenly on top.
Return cake to oven for 5 minutes, until marshmallows are puffed and lightly golden.
In a saucepan over medium heat, melt butter for the frosting, then stir in cocoa powder and milk.
Bring the mixture to a simmer, then remove from heat and whisk in powdered sugar, vanilla, and salt until smooth.
Let cake cool for about 10 minutes, then pour warm frosting over marshmallows and spread evenly.
Allow the cake to cool completely before slicing and serving.