Preheat the oven to 350 degrees and line your 8×8-inch baking pan with parchment paper, creating "handles" that hang over the sides. Then spray with non-stick spray.
In a medium-sized saucepan over medium heat, melt the butter and chocolate, mixing occasionally, until completely melted and smooth. Set aside and allow to cool for 5 minutes.
In a large bowl, whisk together the sugar, eggs, and egg yolk for 3 minutes (this step is crucial). Whisk in the vegetable oil, vanilla extract, and melted chocolate, just until combined.
Gently fold in the flour, sifted cocoa powder, and salt using a silicone spatula. Then mix in 1/2 cup of chopped mini peanut butter cups.
Pour the batter into the prepared baking pan and top with the extra 1 cup of chopped peanut butter cups. Bake for 30-35 minutes, just until the edges are set and the middle of the brownies doesn't jiggle. You can also use a toothpick to check the center (the toothpick should come out clean with just a few crumbs).
Allow the brownies to completely cool (about 1 hour) before slicing them into bars. Keep any leftover brownies in an airtight container on the counter for up to 5 days.