Line a 9x13 inch baking pan with parchment paper.
In a large mixing bowl, combine the melted butter, graham cracker crumbs, powdered sugar, and 1 1/2 cups of peanut butter.
Stir until the mixture is uniform and smooth.
Press the mixture firmly and evenly into the prepared baking pan.
In a microwave-safe bowl, combine the chocolate chips and the remaining 2 tablespoons of peanut butter.
Heat in the microwave in 30-second increments, stirring between each, until completely melted and smooth.
Spread the chocolate mixture over the peanut butter base in an even layer.
Refrigerate for at least 1 hour, or until the chocolate is set, before slicing into 24 bars.