Preheat the oven to 350°F. Grease a 9x13 baking dish, and then lay a sheet of parchment paper across it, making sure to have enough parchment hanging over the sides. This will give you something to hold on to to help pull the baked brownies out of the dish.
In a microwave-safe bowl or dish, add 3/4 cup butter (1.5 sticks) and the chopped semi-sweet chocolate. Microwave the butter and chocolate for 30 seconds at a time, whisking well in between. Repeat this until the chocolate has completely melted. Add the granulated sugar, and whisk until fully incorporated, it's ok if it seems grainy, just make sure it's homogeneous. Add the eggs and 1 tsp of the vanilla extract and whisk until fully mixed.
Sift in the flour and cocoa powder, and add the salt. Using a rubber spatula, fold in the dry ingredients until fully combined; the batter will be quite thick, this is normal. Stir in the chopped peanut butter cups, leaving ~1/3 of a cup left to sprinkle on top of the brownies.
Melt the remaining butter and add it to the bowl of a stand mixer (or use a electric hand mixer) along with the peanut butter, 1 tsp vanilla extract, and powdered sugar. Beat the ingredients together until smooth, scraping down the sides as necessary. Set the peanut butter aside for now.
Take the brownie batter and randomly drop large dollops of the batter into the greased and lined baking dish, leaving ~1/4 of the batter still in the bowl. Next do the same with the peanut butter mix, this time using all of it, spreading it around the dish in spots where there wasn't as much of the brownie mix. With the remaining brownie batter, fill in any empty spots or places where there's a lot of just peanut butter.
Using a knife, cut lines lengthwise back and forth through the batter. This will help swirl and combine the two batters. The degree to which you mix them is up to you, but I recommend not leaving too much of just the peanut butter concentrated in one spot. You can also make your swirl really uniform, or just go crazy! This part is up to you, just make sure the two are well mixed in the pan. Once you're done, sprinkle the top with the leftover peanut butter cups.
Bake the brownies for 30 minutes, and then check the center with a toothpick or cake tester. The brownies are done when the toothpick has a few moist crumbs. If they're not done at 30 minutes, keep testing them every 1-2 minutes (mine took ~35 minutes total).
When done, remove from the oven and allow the brownies to cool completely before cutting. The peanut butter isn't as solid as the chocolate, so these brownies fall apart a bit easier than just regular chocolate ones. For cutting, I recommend using the parchment to pull out the whole brownie sheet as one piece and cutting on a cutting board. Optionally, you can sprinkle the brownies with some flaky sea salt before serving.
Enjoy!
