Line a 9" x 13" baking dish with parchment paper so that the paper comes over the edges of the pan about 1” to make it easy to lift out and cut when finished.
In the bowl of a stand mixer, add the peanut butter and room temperature butter. Using the paddle attachment, cream together until light and fluffy, or about 3 minutes.
Transfer the peanut butter mixture to your prepared baking dish and spread or press evenly over the bottom, then put in the refrigerator to chill while you prepare your chocolate.