Preheat the oven to 350 degrees Fahrenheit.
In a bowl, add the butter for the cookies & the granulated sweetener.
Mix the butter and sugar with an electric mixer.
Next, add the egg and vanilla and mix again.
Add the flour, cocoa powder, and salt to the wet ingredients and mix until combined.
Use a cookie scoop to scoop the batter onto a baking sheet.
Using your fingers, smash the dough down into a flattened circle.
Place the baking sheet into the oven and bake for 13 minutes.
Remove the cookies from the oven and allow the cookies to cool completely while making the filling (approximately 45 minutes)
In a separate bowl, add the filling ingredients: confectioner’s sweetener, butter, and vanilla.
Using an electric mixer, beat the filling ingredients together until combined and smooth.
Once the cookies have cooled completely, add the filling to the inside of 2 cookies.
Repeat until all the cookies are filled.
Store leftover cookies in an airtight container.