Line an 8×8-inch pan with parchment paper and set aside.
In the bowl of a stand mixer fitted with the paddle attachment, add 2 cups creamy peanut butter and 1/2 cup unsalted butter (room temperature). Beat on medium speed until completely combined and smooth.
Add 2 teaspoons vanilla extract and ½ teaspoon kosher salt. Mix briefly to combine, then scrape down the sides and bottom of the bowl.
With the mixer on low speed, slowly add 5 cups powdered sugar. Mix just until the powdered sugar is incorporated and the mixture comes together into large crumbles or a thick dough.
Transfer the peanut butter dough to the prepared pan. Press it evenly into a smooth layer using your hands or the back of a spatula. For a very smooth top, place a piece of parchment over the layer and press down to compact and smooth it.
Place 1-½ cups chocolate chips and 1 teaspoon refined coconut oil in a medium microwave-safe bowl. Microwave in 30-second intervals, stirring between each interval, until the chocolate is fully melted and smooth.
Pour the melted chocolate over the peanut butter layer and spread into an even layer with an offset spatula or the back of a spoon.
If using, sprinkle 2 tablespoons festive sprinkles over the chocolate while it is still wet.
Refrigerate the pan for about 20 minutes, or until the chocolate is set and no longer wet.
Use a sharp, flat-bladed knife to cut the bars into your desired size. For cleaner slices, run the knife under warm water, dry it, and wipe it clean between cuts.
Store the Buckeye Bars in an airtight container in the refrigerator for up to 5 days. Enjoy chilled or at room temperature.