Preheat the oven to 350 degrees. Prepare large-rimmed baking sheets with parchment paper. Set aside.
In a medium bowl, whisk together the flour, cocoa, baking soda, and salt. Set aside.
In a mixing bowl, beat the butter and sugar together until fluffy, about 6 minutes.
Then add the eggs and vanilla, mixing until incorporated and creamy. Scrape down the sides as needed.
Add the flour mixture to the batter and beat just until blended.
Using a medium cookie scoop, drop the batter onto the prepared baking sheets about 2 inches apart.
Bake the cookies for 10 to 12 minutes or until set on the sides and slightly crinkled across the top. Let the cookies cool completely on the baking sheets.
Once the cookies are cool, make the filling by beating the butter until creamy.
Then slowly mix in the confectioner’s sugar, cocoa, vanilla, and salt. Once the dry ingredients are mostly incorporated, increase the speed and beat until whipped and creamy.
Transfer the filling into a piping bag with the end snipped.
Pipe the filling onto a cookie. Then, place another cookie on top, twisting slightly to spread the filling to the edge.
Refrigerate the cookies until the filling has set.
Store in an airtight container in the refrigerator.