Line a 9 x 13-inch baking pan with parchment paper, leaving some overhang for lifting, or lightly grease the pan. Make sure the pan is ready before mixing the base because the mixture sets quickly once combined.

In a large bowl, stir together the melted butter, finely crushed graham cracker crumbs, powdered sugar, and 1 cup of creamy peanut butter. Mix until the color and texture look evenly combined, with no dry pockets of crumbs or visible streaks of peanut butter.

Transfer the peanut butter mixture to the prepared pan and press it firmly into an even layer. Pay special attention to the corners and edges, using the flat bottom of a measuring cup or a spatula to compact the crumbs.

Place the semisweet chocolate chips and the remaining 4 tablespoons of creamy peanut butter in a microwave-safe bowl. Heat in short intervals, stirring after each interval, until smooth; you may also melt the mixture gently in a double boiler.

Spread the melted chocolate mixture over the peanut butter base, guiding it gently toward every corner. Use an offset spatula or the back of a spoon to create a level topping without disturbing the layer underneath.

Refrigerate the pan for approximately 1 hour, or until the bars are firm. Let the chilled slab stand for a few minutes, then lift it out using the parchment and cut it with a sharp knife.