Line an 8"x8" (20 cm x 20 cm) baking pan with aluminum foil or parchment paper, then spray with non-stick cooking spray.
In a medium microwave-safe bowl, melt the butter. Using a rubber spatula, mix in the powdered and brown sugars. Stir in the graham cracker crumbs and then the creamy peanut butter, mixing until smooth and thoroughly combined.
Spread the batter in an even layer in the prepared baking pan, and set aside.
In another bowl, melt the chocolate chips with 2 tablespoons of creamy peanut butter in the microwave in 30-second increments. Stir after each interval until smooth, then spread the chocolate over the peanut butter bars.
Heat the remaining peanut butter in the microwave for 20-30 seconds (this helps with pouring). Drizzle the peanut butter over the chocolate, creating 5-6 straight lines. Rotate the pan 90 degrees (194°F), and using a toothpick, drag it through the lines in both directions to create the pattern.
Cover the pan with plastic wrap and refrigerate for at least 2 hours before cutting.