Preheat oven to 375 degrees Fahrenheit.

In a stand mixer with the paddle attachment, thoroughly mix the flour, cocoa, baking soda, baking powder, salt, and sugar. Add the butter 1 Tbsp at a time until the mixture crumbles, then add the egg. Continue mixing until the dough comes together.

Scoop dough into Tablespoon sized balls and place them on parchment or silicone lined baking sheets 2-inches apart. Slightly flatten dough balls with moistened hands or the bottom of a wet measuring cup.

Bake for 9 minutes. Move baked cookies to a wire rack to cool.

To make the buttercream, whip butter and shortening in a large bowl with a hand mixer until light and fluffy. Mix in the peppermint extract. Gradually beat in the powdered sugar. If the frosting becomes too stiff to pipe from a plastic bag, add water 1 tsp at a time until desired consistency.

Fill a plastic sandwich bag with mint buttercream. Cut off the corner and pipe a swirl on the bottom of one cookie. Top with the bottom of another cookie to create an Oreo sandwich cookie. Repeat with all cookies. Store in an airtight container up to 5 days.