Preheat the oven to 325°F. Grease a 12-well whoopie pie pan (or muffin top pan).
In a mixing bowl, whisk together the our, cocoa powder, baking soda, and salt until blended. In a separate bowl, beat the butter, sweetener, eggs, and extract with a hand mixer until smooth. Stir the wet ingredients into the our mixture. Spoon the batter into the prepared pan, lling each well about two-thirds full. Bake for 12 minutes or until a toothpick inserted into the center of a pie comes out clean. Allow to cool in the pan.
Meanwhile, make the filling: Using the hand mixer, cream the butter, cream cheese, and sweetener in a medium-sized bowl. Add the cocoa powder and stir well. Add the heavy cream to thin it out a little, then add the extract and mix to combine. Set the lling aside.
To make the chocolate drizzle, place the cream, sweetener, and chopped chocolate in a double boiler or in a heat-safe bowl set over a pan of simmering water. Heat on low, stirring, just until the chocolate is melted. Remove from the heat and stir in the extract. Taste and add more sweetener, if desired.
To assemble into whoopie pies, place one pie at side up on a plate. Place 2 tablespoons of filling on the muffin top, then top with another muffin top. Repeat with the rest of the pies and filling. Drizzle the chocolate over each whoopie pie.
Store extras in an airtight container in the refrigerator for up to 4 days.