Prepare an 8-inch by 8-inch square baking pan (or one of a similar size) by greasing it with a little bit of cooking spray and lining it with parchment paper. Set it aside.
Add the peanut butter to a mixing bowl along with the melted butter, powdered sugar and graham cracker crumbs. Mix everything together very well until combined.
Press the peanut butter mixture into the prepared pan and smooth out the top as much as possible.
Chop the chocolate and add it to a separate microwave-safe bowl along with the 3 tablespoons of peanut butter. Microwave on medium power for 30-second increments, stirring every 30 seconds. Continue until melted.
Stir the chocolate and peanut butter together well but gently. Pour the melted chocolate mixture over the peanut butter mixture in the baking pan and smooth out the top.
Refrigerate for 2 hours or pop it in the freezer for about 30 minutes to set.
Once set, use the parchment paper to lift the bars out of the pan, place it on a cutting board and slice into 16 squares.