Make the Oreo cookie crust by crushing the Oreos into a fine crumb using a food processor or blender. Mix the crushed Oreo crumbs and melted butter together in a large bowl. It may look like there is not enough butter, but it will come together. Place into the prepared baking pan, pressing the crumbs down into the pan to create a somewhat even layer.
In the bowl of a stand mixer or using a handheld mixer, mix the peanut butter, butter, confectioners' sugar, vanilla, and salt until the mixture is completely combined and smooth. Spread over the crumb mixture, smoothing the top with an offset or rubber spatula (sprayed with nonstick spray, for ease). Chill the bars for at least an hour, until the filling is set.
Melt the chocolate in medium microwave-safe bowl, stirring every 15 seconds. Or use a double boiler. Pour the chocolate over the peanut butter filling and evenly spread. Chill the bars in the refrigerator for at least an hour or until the chocolate topping has set. Remove bark from the baking pan using the parchment paper and cut into uneven pieces (or use your hands to break it up).
PEANUT BUTTER FILLING