PREP: Line a 9x13-inch pan with parchment paper and set aside. Preheat the oven according to brownie mix instructions.
BROWNIE MIX: Prepare the brownies according to package directions. Pour the mixture into the prepared pan and bake for 2 minutes less than the package directs. Meanwhile, unwrap the peanut butter cups.
PEANUT BUTTER CUPS: Working quickly, press the unwrapped peanut butter cups evenly into the brownies -- 3 rows of 5 each.
FINISH BAKING: Return the brownies to the oven and bake for the remaining 2 minutes. Remove brownies and allow to fully cool at room temperature. If you refrigerate or freeze them, allow them to come back to room temperature before topping with the melted chocolate (otherwise you'll risk streaky/spotty chocolate -- See Note 2).
CHOCOLATE TOPPING: In a microwave-safe bowl, combine the chocolate chips and 2 tablespoons (23g) of shortening or coconut oil. Microwave in bursts of 20 seconds stirring in between the bursts for 20 seconds until the chocolate is completely melted (See Note 3). Melt slowly, stirring well in between bursts to avoid burning chocolate. Use a spatula to scrape every bit of the chocolate on to the cooled brownies. Smooth the chocolate into an even layer.
PEANUT BUTTER: In another small microwave-safe bowl, combine the peanut butter and remaining 1/2 tablespoon (6g) coconut oil or shortening. Microwave for 30 seconds. Remove and stir until smooth. If desired, stir in the vanilla extract. Pour in long stripes over the melted chocolate. Using a knife, marble the top by running the knife horizontally from left to right just through the top chocolate and peanut butter layers. Don't go deep enough to hit the brownies.
ENJOY: Allow brownies to completely set up at room temperature (do not chill!) before enjoying the bars. Cut into bars -- this can get a little messy! I recommend using a very sharp knife and running it under hot water for 10 seconds, drying it off, and then making a cut. Repeat this process for each cut. Use a metal spatula to remove brownies.
STORAGE: These bars are best enjoyed within 3-5 days. Store, covered tightly, at room temperature. These bars do not require refrigeration. The bars can be frozen, but when thawed, they will most likely form white spots on top. They're safe to eat; this is just the milk solids separating and rising.