Pre heat oven to 175°Celsius. Prepare baking trays lined with parchment paper.
Sift the flour, cocoa powder, baking powder, baking soda and salt together and keep them aside.
In a clean bowl, cream butter, peanut butter, and both the white and brown sugars until creamy.
Add the egg and vanilla essence and beat until well incorporated, followed by the sifted dry ingredients. Mix until all the ingredients are well incorporated, and the dough comes together, soft.
Roll the dough and cut out round cookies, approximately 4.5cm in diameter.
Bake the cookies for 9 minutes.
Remove cookies from oven, let them rest in the tray for a few minutes before transfering onto a wire rack to cool completely.
To make the filling, sift the icing sugar into a bowl.
Add the peanut butter and mix until the filling comes together.
Using a ½ tablespoon measuring spoon, scoop the filling out onto a plate. Shape each scoop into round balls.
To assemble the cookies, place one filling ball in the center of one cookie. Flatten lightly with your fingers. Place another cookie on top and press the filling down very gently until it is flat and reaches the edges of the cookies.
Repeat the process until all the cookies are done.
Place the sandwiched homemade peanut butter Oreo cookies in an airtight container. The cookies can last up to 2 weeks if stored properly.