Preheat oven to 350 degrees and spray two 6″ cake pans or one 9″ cake pan with non-stick baking spray.

In the bowl of an electric mixer, beat together the butter and peanut butter on medium-high speed for 2-3 minutes, until light and fluffy. Add the sugars and beat again for another 2 minutes. Add the egg and vanilla and continue to beat until well incorporated and smooth.

In a medium bowl, whisk together the flour, cornstarch, baking soda, and salt. With the mixer on low speed, slowly add the dry mixer to the wet, mixing until just incorporated. Fold in the Oreo pieces with a rubber spatula until just combined.

If you’re doing two 6″ pans, divide the dough in half and press evenly into the pans, and if you’re doing one 9″ pan, transfer it all and press down gently until evenly spread out. Bake for 16-18 minutes, or until edges just start to turn golden brown. Allow the cookie cake to cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.

 for the peanut butter oreo buttercream:

In the bowl of an electric mixer beat together the butter and peanut butter on medium-high speed for 2-3 minutes, until light and creamy. Turn the speed down to low and slowly add the confectioners’ sugar. Once all of the sugar is added, turn up the speed to medium and beat for a minute or so. Add the milk and salt and continue beating for another 2-3 minutes until smooth. Fold the crushed Oreos in with a rubber spatula and mix until well incorporated.

Transfer buttercream to a piping bag fitted with a large round tip (or star if you want to be fancy) and pipe a border along the edge of your cooled cookie cake(s). Melt peanut butter chips and drizzle with a spoon or fork. Decorate with the quartered Oreos and store in the fridge until ready to serve.

Source: http://www.ericasweettooth.com/