Preheat oven to 350°F (175°C) and line baking sheets with parchment paper.
In a large bowl, cream together butter and sugar until light and fluffy.
Mix in the egg and vanilla extract.
In another bowl, whisk together flour, cocoa powder, baking soda, and salt.
Gradually add the dry ingredients to the wet ingredients and mix until combined.
Scoop dough onto baking sheets, flatten slightly, and bake for 8-10 minutes.
Let cookies cool completely before adding filling.
For the filling, beat butter until creamy, then gradually mix in powdered sugar, mint extract, and food coloring.
Fold in mini chocolate chips.
Spread filling onto half of the cookies, then top with the remaining cookies to make sandwiches.