Prepare the Pan: Line a 9x13 inch baking pan with parchment paper, leaving an overhang on all sides. This will make it easier to lift the bars out later. Lightly grease the bottom if desired to help the parchment stick.
Mix the Base: In a large bowl, combine the finely crushed graham cracker crumbs, melted butter, powdered sugar, and creamy peanut butter. Stir well with a spoon or spatula until all ingredients are thoroughly combined and form a thick, cohesive mixture.
Press into Pan: Press the peanut butter mixture evenly into the prepared baking pan. Use the back of a spoon or your hands to press it down firmly and create a compact, even layer. Set aside.
Melt Chocolate Topping: In a microwave-safe bowl, combine the chocolate chips, 4 tablespoons of butter, and optional shortening or coconut oil. Microwave in 30-second intervals, stirring after each, until the mixture is smooth and fully melted. Be careful not to overheat the chocolate.
Pour Topping: Pour the melted chocolate mixture evenly over the peanut butter base. Use an offset spatula or the back of a spoon to gently spread it from edge to edge, ensuring full coverage.
Chill and Set: Place the pan in the refrigerator for at least 2 hours, or until the chocolate topping is completely set and firm. For the cleanest cuts, it's crucial that the bars are thoroughly chilled.
Cut and Serve: Once firm, use the parchment paper overhang to lift the entire slab of No-Bake Chocolate PB Bars out of the pan. Place on a cutting board. Use a sharp knife, warmed under hot water and wiped dry, to cut the bars into desired squares or rectangles. Serve immediately or store as directed.