blend butter and sugar. add egg, vanilla, milk and remaining ingredients. scoop out cookies with mini ice cream scoop. bake at 350 degrees for 5-7 minutes. remove from oven.
cut marshmallows into thirds. push a third of a marshmallow into the center of each cookie. bake for 2 more minutes. remove from oven and pat down the marshmallows. let cool and frost.
2 cups powdered sugar
1/2 teaspoon salt
5 tablespoons milk
3 tablespoons cocoa
3 tablespoons melted butter
whisk all ingredients together until smooth.