Preheat oven to 350 degrees. Line a cookie sheet with parchment paper.
Add the butter and sugar to a mixing bowl and cream together until light and fluffy, about 3 minutes. Add eggs and vanilla and mix well.
In a separate bowl, combine the flour, cocoa powder, baking soda, and salt. 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.
For the frosting:
Add butter to a mixing bowl and 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.
Once cookies have cooled, cover the tops of the marshmallow and cookie with frosting.
Store leftovers in an airtight container, or freeze for later.
My Great-Aunt always added a walnut on top at the end. You could also add crushed peppermint candy, to make them festive for Christmas.Storing and Freezing Instructions:Once cookies have cooled completely, store them in an airtight container on the counter for 3-4 days. To freeze place them in a freezer safe bag or container and freeze for 2-3 months.