Line an 8x8-inch baking dish with parchment paper, leaving overhang on two sides for easy removal.
In a large bowl, mix the peanut butter, melted butter, powdered sugar, and vanilla extract until smooth and creamy.
Press the peanut butter mixture evenly into the prepared baking dish.
In a microwave-safe bowl, combine the chocolate chips and coconut oil. Heat in 30-second intervals, stirring between each, until fully melted and smooth.
Pour the melted chocolate mixture over the peanut butter layer, spreading it into an even layer with a spatula.
Refrigerate for at least 2 hours, or until set.
Use the parchment paper to lift the bars out of the pan. Cut into 16 squares and serve.
Notes
Store leftovers in an airtight container in the refrigerator for up to 1 week. For longer storage, freeze for up to 2 months.