Preheat oven to 350°F. Line cookie sheets with parchment paper or silicone baking mats.

In a medium bowl, whisk together cocoa powder, baking soda, salt, and flour. Set aside.

In the bowl of an electric mixer fitted with the paddle attachment, cream butter and both sugars. Mix in egg and vanilla and mix until smooth. Add dry ingredients and mix until combined.

Scoop 1 or 2 tablespoon sized balls of dough onto the cookie sheets. Bake for 8-9 minutes (small) or 10-11 minutes (large) until they just lose their glossy sheen. Err on the side of underdone for these cookies; they taste better that way. Let cool completely before filling and making sandwiches.

To make the filling: beat butter until smooth in a large bowl with a hand mixer. Mix in salt, then slowly beat in powdered sugar. Add 1/4 teaspoon peppermint extract, then 1 tablespoon of cream, adding more as needed for spreadable consistency. Taste to make sure the mint flavor is to your liking, and if necessary, add more extract. Mix in the mini chocolate chips.

Frost half of the cookies with about 2 tablespoons of frosting and sandwich with the other half. You will have just enough frosting to do all the cookies.