Using a stand mixer, beat together the butter, sugar, eggs and vanilla until the mixture is light and fluffy.
Add in the flour, cocoa, baking soda and salt. Stir the mixture until it is well blended.
Add in the Mounds bars and chocolate chips. Mix until combined.
Drop the batter by rounded tablespoons onto greased baking sheets, making 36 cookies. Press mini chocolate chips into the top of the cookies (if desired) for a pretty look.
Bake the cookies in a 350 degree oven for 8-10 minutes. Do not overbake. Let the cookies cool for 5 minutes on the baking sheet, then move them to a wire rack to cool completely.
Make the buttercream:
For the buttercream, put the butter in the bowl of a stand mixer. Whip the butter on medium speed for 2-3 minutes until the butter is almost white.
Add the coconut extract and mix again.
Add the powdered sugar, one cup at a time, to the butter mixture, mixing as you add the sugar.
Add the heavy cream a tablespoon at a time until the buttercream is at the consistency that you'd like.
Whip the frosting for 2-3 minutes until it is light and fluffy.
Assemble the cookie sandwiches:
Spread the frosting on the bottom of a cooled cookie and top with another cookie.
Store in an airtight container.
This recipe makes 18 sandwich cookies.
Notes
The calories shown are based on the recipe making 18 large cookie sandwiches, with 1 serving 1 cookie sandwich. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate. **We are not dietitians and recommend you seek a nutritionist for exact nutritional information.**