Preheat the oven.

In a bowl, cream together the softened butter, granulated sugar, and brown sugar.

Beat in the eggs one at a time, then stir in the vanilla extract.

In another bowl, combine the flour, cocoa powder, baking soda, and salt.

Gradually blend the dry ingredients into the wet mixture.

Stir in the chocolate chips and mix well.

Shape the dough into balls and place them on a baking sheet.

Bake in the preheated oven for 10 minutes.

Let cool, then prepare the filling by mixing powdered sugar, softened butter, peppermint extract, milk, and green food coloring if desired.

Spread filling between two cookies to make sandwiches.