Use either an 8x8 or 9x9 square pan. If you want the bars to come out easily for cutting and serving, line the pan with foil, leaving enough over the sides to lift out.
In a food processor, blend Oreos until fine crumbs form. Add in melted butter and blend until combined. Pour into pan and press down to evenly cover the bottom of the pan to form a crust.
In a medium bowl, add the room temp cream cheese, powdered sugar, peanut butter, and vanilla extract. Beat with an electric hand mixer until smooth and creamy. Taste for sweetness, add additional sugar if desired, but I like the slightly sweet, tart flavor of a cheesecake.
Pour into the pan over the crust, spread evenly. Refrigerate at least 3-4 hours, or overnight to set. Garnish as desired. Slice and serve.