In a small saucepan over low heat, combine the chopped chocolate and butter. Stir until melted and smooth. Remove from heat and let cool to lukewarm.
In a large bowl, beat the eggs with the sweetener, espresso powder, vanilla extract and salt until thickened, 3 to 5 minutes. Beat in the melted chocolate.
All at once, add the almond flour, cococa powder, and baking powder and mix until well combined. Cover and refrigerated 1 hour.
Preheat the oven to 350F and line a large baking sheet with a silicone mat or parchment paper.
Using about 1 ½ tablespoon of dough, roll into balls and place on the prepared baking sheet. Bake 5 minutes, then remove from the oven and gently press down to about ½ inch thick with a flat bottomed glass. Cover the bottom of the glass with parchment paper if the cookies are sticking.
Bake another 12 to 15 minutes, until just barely firm to the touch. Remove and let cool completely on the pan.
Make sure the butter is very well softened. Place in a medium bowl and beat until smooth. Beat in the powdered sweetener, espresso powder, and vanilla extract.
Beat in the melted chocolate until smooth.
Take one cookie and pipe or spread the bottom with the filling. Top with another cookie. Repeat with the remaining cookies and filling.