In a large bowl, add the butter and sugar and cream together with a hand mixer. Add the eggs and vanilla extract to the creamed mixture and mix until combined. In a medium bowl, mix the cocoa powder, flour, baking soda, and salt. Slowly add the dry ingredients and continue mixing until incorporated.
Roll the dough into 2 logs that are about 2-inches high and 1-foot long. Wrap them in waxed paper and place in the refrigerator for 2 hours.
Preheat oven to 350 degrees F.
Once thoroughly chilled, slice the cookies into ½-inch thick rounds and cover with sanding sugar. Place on a parchment paper lined cookie sheet and bake for 10 minutes.
Transfer to a wire rack to cool.
To make the filling, add the butter, marshmallow cream, and mint extract. Use an electric mixer to beat until thoroughly combined and add green food coloring according to your desired shade of green. Spread about a tablespoon of the filling on the bottom side of one of the cookies and then place the other cookie on top. Repeat for all of the cookies.
Makes about 24 sandwich cookies.
Recipe by The Daring Gourmet at https://www.daringgourmet.com/chocolate-marshmallow-mint-sandwich-cookies/