Preheat your oven to 350°F (175°C). Line your baking sheets with parchment paper to ensure your cookies don’t stick.
In a medium bowl, whisk together the all-purpose flour, cocoa powder, baking soda, and salt.
In a large mixing bowl, cream together the 1/2 cup of room temperature butter, granulated sugar, and brown sugar until light and fluffy.
Beat in the large egg and 1 teaspoon of vanilla extract until well combined.
Gradually add the dry mixture to the wet mixture, mixing until just combined.
Using a cookie scoop or a tablespoon, drop dough onto the prepared baking sheets, leaving enough space between each cookie.
Bake for 10-12 minutes, or until the edges are set and the centers are slightly soft.
Let the cookies cool on the sheets for 5 minutes before transferring them to a wire rack to cool completely.
While the cookies are cooling, prepare the filling by beating together the 1/4 cup of butter, 1/4 cup of vegetable shortening, powdered sugar, and 1 teaspoon of vanilla extract until smooth.
Once the cookies are completely cool, spread a generous amount of filling on one cookie and sandwich it with another.
