Line a 9×13-inch baking pan with parchment paper, leaving enough parchment overhang to lift the bars out of the pan. Set aside.

Place half (8 ounces/227g) of the chopped white chocolate and 1/2 teaspoon of oil in a medium microwave-safe bowl or liquid measuring cup. Melt in 20-second increments, stirring with a silicone spatula or spoon after each increment, until completely melted and smooth. White chocolate can overheat easily, so it’s important to do this in increments. Pour the melted white chocolate into the prepared pan and spread it evenly with an offset spatula to cover the bottom of the pan. Place the pan, uncovered, in the refrigerator to chill for 10–15 minutes, to soft-set the white chocolate.

In a medium bowl with a hand mixer or in the bowl of a stand mixer fitted with the paddle attachment, beat the peanut butter and butter until smooth and creamy, about 2 minutes. Scrape down the sides and up the bottom of the bowl as necessary. Add the confectioners’ sugar, vanilla, and salt, and beat until combined, about 1 minute. Spoon dollops of the peanut butter mixture onto the bottom layer of white chocolate, and spread it evenly with an offset spatula as best you can. Place the pan back in the refrigerator while you melt the remaining white chocolate.

Repeat step 2 to melt the remaining white chocolate and 1/2 teaspoon oil. Pour and spread it evenly with an offset spatula to cover the peanut butter layer. Sprinkle chopped Reese’s Pieces on top.

Place the pan in the refrigerator, uncovered, for at least 30 to 60 minutes to set the white chocolate. (You can also leave it in the refrigerator for up to 1 day, but cover it loosely after an hour. If refrigerating it for more than 1 hour, you may notice the top white chocolate layer cracks when you’re cutting the bark into pieces. This is normal and expected, because the layer is quite cold.)

Lift the bark out of the pan by holding onto the edges of the parchment paper, and place it on a cutting board. Use a sharp knife to cut the bark into pieces.

Cover and store bark in the refrigerator for up to 1 week, or in the freezer for up to 3 months.