Cream together the butter and sugar using a stand or a handheld mixer on medium.

While still mixing on medium, add in the egg and vanilla extract.

Turn the mixer on low and add in the flour ½ cups at a time.

Add in the cocoa powder, salt, baking soda, and cornstarch.

Cover the mixing bowl with plastic wrap and let chill in the refrigerator for 20 minutes.

Preheat oven to 350 degrees and line a cookie sheet with parchment paper or a silicone baking mat.

Spoon the cookie dough onto the paper or baking mat and bake for 9-10 minutes.

Let cool on the cookie sheet for 2 minutes, then transfer to a cooling rack.

While the cookies are cooling, prepare the cream filling by whipping the butter and shortening on medium speed with the whisk attachment for 1-2 minutes.

Add in the powdered sugar ½ cup at a time, then the vanilla extract.

Using a pastry knife or a butter knife, spread the cream filling on the bottom of one cookie, then place the other cookie on top.