Preheat the oven to 350. Line two baking sheets with parchment or silicone liners.

In a large bowl, whisk together the butter and brown sugar. Add the vanilla and egg, and mix to combine. Add the flour, cocoa powder, salt, baking soda, and baking powder, and mix until combined.

Spoon the dough by tablespoons onto the prepared baking sheets. Flatten slightly with a glass sprayed with cooking spray. Bake 11-13 minutes, or until the tops are dry and starting to crack. Remove and let cool completely.

To make the frosting, in a medium bowl, beat together the butter and peanut butter until light and fluffy. Add the powdered sugar and enough half and half to make a light and fluffy frosting.

To assemble, spread half of the cookies with a thick layer of frosting and top with the remaining cookies.