Preheat oven to 350.

With the paddle attachment on your stand mixer, or with a handheld mixer combine the butter and sugar and beat until it looks light and fluffy.

Add in the egg and vanilla and beat until smooth.

In a separate bowl, combine cocoa, flour, baking soda, baking powder, and salt.

While mixing, slowly add in the flour mixture and mix until the dough comes together and no dry spots remain.

Spoon cookie dough onto lined baking sheets and bake for 5 minutes; the cookies will be underdone and look just barely set.

Press a marshmallow into the tops of each cookie and put the cookies back into the oven for an additional 3 minutes or until the marshmallow has melted a bit and started to spread. Remove from oven and let cool completely.

For the Frosting

Combine melted butter with the cocoa powder and mix until smooth. Slowly mix in milk and vanilla until combined. Stir in powdered sugar and mix until smooth and creamy.

When cookies are completely cooled, spread a layer of frosting on each one.