Line a 9x13 inch baking pan with parchment paper, leaving overhang on sides. Lightly spray with cooking spray.
In a large bowl, combine graham cracker crumbs and powdered sugar. Add melted butter, peanut butter, and vanilla. Mix until thick and moldable.
Press mixture firmly into prepared pan in an even layer. Use the back of a spoon to pack it down tightly.
Melt chocolate chips with 3 tablespoons peanut butter in microwave in 30-second intervals, stirring between each, until smooth.
Pour melted chocolate over peanut butter layer and spread evenly. Sprinkle with Reese's Pieces if using.
Refrigerate for at least 2 hours until completely set. Cut into bars and serve.