Line an 8x8-inch baking dish with parchment paper, leaving some overhang for easy removal.
In a medium bowl, mix the graham cracker crumbs, melted butter, peanut butter, and powdered sugar until well combined.
Press the mixture evenly into the bottom of the prepared baking dish.
In a microwave-safe bowl, combine the chocolate chips and vegetable oil. Microwave in 30-second intervals, stirring between each, until fully melted and smooth.
Pour the melted chocolate over the peanut butter layer and spread it evenly with a spatula.
Refrigerate for at least 2 hours, or until the chocolate is set.
Once set, lift the bars out of the dish using the parchment overhang. Cut into 16 squares and serve. Store leftovers in the fridge!