To Make the Brownies:

1. Preheat your oven to 350°F and liberally grease a 13"x9" baking pan with cooking spray.
2. Prepare the brownie mix according to package directions and pour the mixture into the prepared pan.
3. Bake for approximately 20-22 minutes or until a toothpick inserted near the center comes out mostly clean and with light crumbs. Cool the brownies completely in the pan.

For the Buckeye Filling:

1. In the bowl of a stand mixer, cream together the butter, 1/2 cup of the peanut butter, and the vanilla together until blended, about 2 minutes.
2. Gradually add the powdered sugar, about one cup at a time, until filling comes together in a ball.
3. Add in the milk/cream if needed to thin the filling out into a smooth, spread-able consistency.

To Assemble the Buckeye Brownies:

1. Spread the filling over top of the cooled brownies in an even layer. Refrigerate for 15 minutes. While the mixture chills, prepare your final ganache topping.
2. In a medium-sized microwaveable bowl, melt together the chocolate chips and remaining 1/2 cup of the creamy peanut butter for 30 seconds on HIGH power. Stir, then microwave for another 30 seconds. Stir again to melt the remaining chips and combine the mixture into a cohesive ganache.
3. Spread the ganache over top of the bars in an even layer.
4. Refrigerate the bars for 15-20 minutes or until ganache is set.
5. Cut into squares and serve immediately. Store leftovers airtight in the fridge.