Cut butter into chunks and place in a large microwave-safe bowl. Microwave for about 1 minute or until butter is melted.
Stir in powdered sugar, graham cracker crumbs, and peanut butter. Stir well until all of the ingredients are incorporated.
Line a 9 x 13 inch dish with foil. Drape the foil over the sides of the dish so that you can easily remove it once the bars set. Lightly spray with non-stick spray.
Press the peanut butter mixture into the bottom of the dish.
Melt the semi-sweet chocolate according to package directions. Once melted, spread the chocolate over the peanut butter mixture.
While the chocolate is still warm, cut the bars. Allow to set for at least an hour before serving.