First, make your cookies! Preheat the oven to 350° F and line two cookie sheets with parchment paper. Grease the parchment with cooking spray and set aside. In a small bowl, whisk together the egg whites and vanilla until combined and frothy. Then, in a large bowl, whisk together the cocoa powder, sifted powdered sugar, salt, and espresso powder. Pour the egg whites and vanilla into the dry ingredients and stir together with a spatula or wooden spoon - it will seem very dry but keep stirring until it's thick and glossy. Add in your chopped roasted peanuts and your chocolate chips and stir to combine. Then scoop out tablespoons of the cookie dough and place mounds on your cookie sheet (should fit about 12 per sheet).
Important step! Let these dough mounds sit for 30 minutes before baking! If you do not let your cookies sit, they will spread way too thin and not be usable for sandwiching. Once the cookie dough mounds have sat for 30 minutes at room temperature, bake for 10-11 minutes, then remove and let cool completely before filling.
While the cookies are cooling, make your buttercreams. In a medium bowl, make your peanut butter buttercream. Using an electric mixer, beat the butter and peanut butter until smooth. Add the vanilla and powdered sugar and continue to beat until combined. Scoop buttercream into a pastry bag (either with a large round tip or just cut off a half-inch opening in the bag to pipe. Similarly, in a medium bowl, make your chocolate buttercream. Beat the butter, cocoa powder, vanilla, and powdered sugar until smooth, then scoop into a pastry bag and prepare the same way as the peanut butter.
Once cookies are cool, flip over and pipe peanut butter and chocolate buttercreams onto half of the flipped over cookies (we piped a chocolate dot in the center and a peanut butter ring around that, do it however you like!), then pair the filled halves with the empty cookie halves to create sandwiches. Press down gently to fit the cookie halves together and then serve!