Preheat oven to 350 degrees. Line a cookie sheet with parchment paper.

Wet Ingredients: Add the butter and sugar to a mixing bowl and cream together until light and fluffy, about 3 minutes. Add eggs and vanilla then mix well.

Dry Ingredients: In a separate bowl, combine the flour, cocoa powder, baking soda, and salt.

Combine: Add to the bowl with the creamed butter mixture and stir just until combined. Scoop the dough by rounded tablespoons onto your cookie sheet.

Bake for 8 minutes. Remove from oven and top with a marshmallow half (cut-size down). Return to oven for 2 minutes. Allow to cool completely while you make the frosting.

Frosting: Add butter to a mixing bowl then beat with electric mixers for 1 minute. Add the cocoa powder and milk and mix to combine. Add powdered sugar and mix for at least 2-3 minutes or until smooth. Mix in vanilla. Add more powdered sugar if needed, to reach a smooth, spreadable frosting consistency.

Frost: Once cookies have cooled, cover the tops of the marshmallow and cookie with frosting.

Store leftovers in an airtight container, or freeze for later.