Line an 8x8-inch pan with aluminum foil or parchment paper and set it aside.
In a medium-bowl, mix the melted butter, graham cracker crumbs and powdered sugar. Stir in 1 cup of peanut butter, then press the mixture evenly into the prepared pan.
In a small bowl, melt the remaining 3 tablespoons of peanut butter with the chopped chocolate in the microwave or on the stove in a double boiler. Stir until smooth. Spread evenly over the peanut butter layer.
Chill the bars in the refrigerator until completely firm, at least 2 hours. Allow them to sit at room temperature for 10 minutes before cutting. Serve chilled.
Cover the leftover bars tightly and refrigerate for up to 1 week.
Notes
Setting these bars out for a few hours at room temperature for serving is okay.