Preheat the oven to 350ºF, and line two baking sheets with parchment paper.

In a stand mixer fitted with the paddle attachment, beat the butter and cream cheese together until fluffy. Add the egg and cake mix, and continue beating until fully incorporated.

Scoop cookie dough out onto baking sheets, and bake for 10-12 minutes or until they look dry and set on the surface. Allow cookies to cool about 5 minutes before using the back of a cookie scoop/spoon to press a "well" into the center of each cookie. Allow to cool completely.

In a stand mixer fitted with the paddle attachment, beat the butter and salt together until light and fluffy. Add the powdered sugar, a half cup at a time, beating until combined. Add the mint extract, milk, and food coloring, and continue to beat until fluffy.

Transfer buttercream to a piping bag fitted with a round tip (or a ziplock bag with the corner cut off), and pipe buttercream into the "wells" of each cookie.

Place the chocolate and coconut oil in a medium microwave-safe bowl, and heat in the microwave for 1 minute. Remove and stir until smooth. If still lumpy, return to the microwave in 15-second increments until completely smooth.

Using a small spoon, ladle about 2 teaspoons of the melted chocolate on top of the cookie. Using the back of the spoon, spread the chocolate around until it covers all of the buttercream. Repeat with the remaining cookies.