Peanut Butter Layer:
Prepare the Baking Dish:
Line a 9×13-inch baking dish with parchment paper, leaving some overhang for easy removal. Set aside.
Mix the Peanut Butter Layer:
In a stand mixer or a medium-sized mixing bowl, beat the softened butter on medium-high speed for 1 minute until smooth and creamy.
Add the peanut butter and vanilla extract, and continue mixing for another minute until well combined.
Lower the mixer speed to medium-low, and gradually add the powdered sugar, 1 cup at a time, mixing well after each addition.
Once all the powdered sugar is incorporated, increase the mixer speed to medium-high and mix until the peanut butter mixture is smooth and fully combined.
Spread the Peanut Butter Layer:
Using a silicone spatula or an offset spatula, evenly spread the peanut butter mixture in the bottom of the prepared baking dish.
Chocolate Ganache:
Make the Ganache:
In a small microwave-safe bowl, combine the chocolate chips and vegetable oil. Microwave for 1 minute, then stir. Continue heating in 15-second intervals, stirring after each interval, until the chocolate is completely melted and smooth.
Spread the Ganache:
Pour the melted chocolate ganache over the peanut butter layer and use a silicone spatula or offset spatula to spread it evenly across the top.
Chill and Slice:
Cover the baking dish and refrigerate for 15-20 minutes to allow the chocolate to set slightly.
Once set, remove the dish from the refrigerator and carefully slice the buckeye bars into 15 squares (approximately 2 ½ inches by 3 inches).
Cover the dish and return the bars to the refrigerator to chill for an additional 1 hour 45 minutes.
Serve:
To serve, remove the bars from the baking dish using the parchment paper overhang. You may need to carefully re-slice the bars if necessary. Serve immediately and enjoy!