Add the flour, cocoa powder, baking soda, and salt to a large bowl. Set aside.

Add the butter to a mixing bowl and beat on high speed until creamy. Add granulated sugar and brown sugar and mix until well combined. Mix in egg and vanilla extract.

Add the dry ingredients and mix on low speed until combined. Cover the dough and chill for 30 minutes.

Preheat oven to 350 degrees F.

Remove the cookie dough from the refrigerator. Line two large baking sheets with parchment paper.

Roll the dough into small, 2 teaspoon sized balls. You should be able to roll about 30 cookie dough balls. I can fit 15 on each of my cookie sheets. Press down lightly on the top of the balls to slightly flatten. Bake for 6-8 minutes. The will appear soft but will continue to harden as they cool. Cool for 5 minutes on the baking sheet, and then remove to a wire cooling wrack to cool completely.

Beat together butter and shortening until smooth. Add powdered sugar and vanilla and mix for 1-2 minutes, until smooth.

Once the cookies have cooled, add a small spoonful of filling onto the inside of a cookie. Place the inside of another cookie on top and press together.

Cookies store well, covered in the fridge, for up to 1 week. They also freeze well, up to 2 months. Filling can be made up to 1 week ahead of time, stored in the fridge. Allow to come to room temperature before filling cookies.