Grease a 9 x 13 inch pan and preheat the oven to 350 degrees.
Prepare the brownies according to package instructions, pour into the pan, and bake for 16-17 minutes, then remove.
Press the peanut butter cups (unwrapped) into the brownies and return to the oven for another 5-6 or so, or until an inserted toothpick comes out clean. Remove and let cool.
Combine the chocolate chips and 2 Tablespoons of shortening in a microwave safe bowl and heat in 30 second increments at a time, stirring in between, until melted, and spread evenly over the brownies.
Combine the peanut butter and the remaining ½ Tablespoon of shortening and microwave until melted, 20-30 seconds. Stir in the vanilla.
If the peanut butter is very melty, let it cool for a moment, then pour in long strips over the brownies.
Run a clean butter knife horizontally from left to right, just through the top of the chocolate and peanut butter laters. Clean your knife in between each run. If the top is too “peanut buttery” looking, you can dig the knife down until it hits the chocolate layer and pull that layer back up over the peanut butter.