Preheat the oven to 350°F (175°C).

In a mixing bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy.

Add the eggs and vanilla extract, mixing until well combined.

In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

Fold in the chocolate chips.

Drop tablespoon-sized balls of dough onto a baking sheet lined with parchment paper.

Bake for 10-12 minutes, then allow to cool completely.

In a bowl, beat together the powdered sugar, softened butter, peppermint extract, and milk until smooth and creamy.

Add food coloring if desired to achieve a mint green color.

Spread a generous amount of mint filling on the flat side of one cookie and top with another cookie to create a sandwich.

Repeat with remaining cookies.