For the cookies, preheat oven to 300F and line a baking sheet with parchment paper or silicone mat.
In a large bowl, whisk together almond flour, peanut flour, erythritol, baking powder and salt. Stir in melted butter and melted peanut butter. Then stir in egg, vanilla and stevia until dough comes together.
Turn out dough onto a large sheet of parchment paper and pat into a rough circle. Cover with another sheet of parchment and roll out to 1/4 inch thick.
Use a 2½ inch round cookie cutter to cut out cookies, and gently lift with an offset spatula or sharp knife to transfer to prepared baking sheet. Re-roll scraps of dough and cut out more cookies until no more can be cut. You should get about 24 cookies, depending on the size of your cookie cutter.
Bake 18 minutes, or until browned around the edges and just firm to the touch. Remove from oven and let cool completely (they are fragile when warm).
For the frosting, beat together melted butter and cocoa powder until combined. Beat in powdered erythritol until thick and combined.
Beat in vanilla, then beat in heavy cream, one tablespoon at a time, until a spreadable consistency is achieved. How much cream will depend upon your cocoa powder and how absorbent it is.
Pipe or spread frosting on the bottom of one cookie and then top with another cookie, bottom-side down.
Serves 12. Each serving has 6.9 g of carbs and 3.2 g of fiber. Total NET CARBS = 3.7 g.