In a bowl mix together cocoa powder, flour, baking powder, baking soda and salt with a spoon. In a separate large bowl using a hand mixer cream together butter, sugars, egg and vanilla.
Add dry mixture to wet mixture and continue to mix until combined.
Cover cookie dough with plastic wrap and refrigerate for 1 hour.
Preheat oven to 350 degrees. Roll the dough into large teaspoon size balls.
Place balls on cookie sheets that have been sprayed with nonstick spray, or put down silicone baking mats. Once balls are down use a spatula to gently flatten them down.
Bake cookies for 8 minutes.
Cream Filling
In a bowl use a hand mixer combine the butter and peanut butter until creamy.
Add powdered sugar and milk and continue to mix until combined.
To Make Cookies
Take one cookie and put filling on it. Then take another cookie and put it on top. Flatten down with your hand slightly to make the cookie sandwich.