Prepare a 9x13 baking dish or equivalent with cooking spray or parchment paper. In a large bowl, mix the butter, graham cracker crumbs, peanut butter, and powdered sugar until smooth. Press into the bottom of the prepared pan.
Place the chocolate chips in a microwave-safe bowl. Microwave for 30 seconds, then stir. Repeat 3 or 4 times until the chocolate is totally smooth. Pour the chocolate over the peanut butter mixture and smooth into an even layer.
Chill the bars for 1 hour or for a few days, covered, in the fridge, then cut into bars and serve.