In the bowl of your stand mixer with paddle attached, or a large bowl with a hand mixer, add your butter and sugar and beat together until smooth and creamy.
Add your vanilla and egg and mix until the egg has become homogenous in the cookie dough. Remember to scrape down the sides of the bowl to make sure all the butter has been mixed into the egg.
Add your cocoa powder and mix on low until all of the dough is dark with no dry cocoa powder remaining.
Add your flour and mix on low until the dough starts for form large chunks and there are no dry spots.
Transfer all the dough to a sheet of parchment paper or plastic cling wrap and form into a disc like shape.
Wrap the dough up and chill in the refrigerator for at least one hour, however overnight is fine as well.
When the dough has chilled, flour a work space generously and remove your dough from the refrigerator. Allow the cookie dough to sit for 5-10 minutes.
Roll out your dough to about ¼ inch thick but no thinner than ⅛ inch thick.
Use a round cookie or biscuit cutter to cut as many cookies out. I used a 2½ inch cutter and was able to get about 36 cookies. Try to cut as close together as possible to avoid re-rolling. You can gather the scraps and re-roll the dough, however any cookies not from your first roll with probably have a slightly different texture due to the additional flour and the extra working of the dough. Place your cut out cookies on parchment lined baking sheets and chill in the refrigerator for at least 30 minutes but preferably an hour.
Preheat your oven to 350°F before your cookies come out of the refrigerator.
Make sure your cookies have 1-2 inches separating each cookie. Bake on the center rack for about 12-14 minutes. Cookies will be hard to tell browning because of the chocolate, but if your cookies are thicker I would bake more towards 14 minutes. Additionally if you would like the cookies to have a little more crispness I would use the longer bake time as well. I baked sheets at both 12 and 14 minutes and none were over or under done. Remove from the oven and transfer to a wire cooling rack. Let the cookies completely cool before adding frosting and making sandwiches.
In a medium large bowl, add butter and peanut butter and mix together with a hand mixer on medium until smooth (about 1 minute)
Add powdered sugar and mix on low until frosting comes together. It will be thick and a little dry looking.
Add heavy cream and blend until smooth.
Frost one half of each cookie round either with a knife or a piping bag, and then sandwich together. Store in an airtight container for up to 3-4 days. Enjoy!
