Prepare the Pan:
Line a 9×13-inch baking dish with parchment paper. Set aside.

Make the Peanut Butter Layer:
In a mixing bowl or stand mixer, beat softened butter on medium-high speed for 1 minute until smooth.
Add peanut butter and vanilla extract; continue mixing for another minute until creamy.
Reduce the mixer speed to medium-low and gradually add powdered sugar, 1 cup at a time, mixing after each addition.
Increase the mixer speed to medium-high and beat until the mixture is completely combined and smooth.
Spread evenly into the prepared baking dish using a silicone or offset spatula.

Prepare the Chocolate Ganache:
Combine chocolate chips and vegetable oil in a microwave-safe bowl.
Heat for 1 minute, stir, then continue heating in 15-second intervals, stirring between each, until smooth and fully melted.

Layer and Chill:
Pour and spread the chocolate ganache evenly over the peanut butter layer.
Cover and refrigerate for 15–20 minutes, just until the chocolate begins to set.

Slice and Finish Chilling:
Remove from the refrigerator and carefully slice into 15 bars (approximately 2½ × 3 inches).
Cover again and refrigerate for an additional 1 hour and 45 minutes until fully set.

Serve:
Use the parchment paper to lift the bars from the pan. If needed, re-slice before serving. Enjoy chilled or at room temperature.