Preheat oven to 350°F. Grease and line a 8″x 8″ pan with parchment paper and set aside.
In a large microwave-safe bowl, combine butter and sugar. Microwave for 30 seconds, then remove and stir with a whisk. Repeat process 2 more times. Though granules of sugar will remain, the goal is to get the mixture to a glossy finish.
Allow mixture to cool slightly, then add in eggs one at a time, whisking in between each addition. Whisk until the mixture lightens in color slightly.
Add in cocoa powder, salt, and vanilla extract. Fold in flour until just combined. Last, fold in the peanut butter cups.
Pour mixture into prepared baking pan and bake for roughly 35 minutes, testing for doneness by inserting a knife into the center of the brownies beginning at 30 minutes. You’ll know they’re done when the edge is set, but a knife inserted into the middle still comes out with a bit of brownie batter on it. It might take up to 55 minutes for them to cook through depending on your oven!
Notes
Make sure you’re using a high quality cocoa powder! We usually make these with Rodelle’s Dutch processed cocoa powder, but I have also used natural cocoa and other brands. I prefer to use Dutch processed as it has a smoother, deeper flavor, but natural will also work if it’s all you have on hand.
Be careful not to over bake your brownies. When you insert a knife into the middle you’ll want it to come out a little gooey as the brownies will continue to bake as they cool.