Preheat your oven to 350°F. This step is crucial as it ensures even baking. While the oven is heating up, grab an 8-inch square baking pan and spray it with nonstick cooking spray to prevent sticking.
In a microwave-safe bowl, combine semi-sweet chocolate chips and unsalted butter. Microwave for about 45 seconds, then give it a good stir. If the mixture isn’t mostly melted, microwave it for an additional 20 seconds. Stir until the mixture is completely smooth.
Once your chocolate and butter mixture is ready, whisk in the cocoa powder and instant espresso powder. Mixing these dry ingredients into the warm chocolate will help them dissolve better, enhancing the flavor.
In a separate bowl, whisk together the eggs, granulated sugar, vanilla extract, creamy peanut butter, and salt. You want everything to be well combined; this will create a smooth batter.
Now, gradually whisk the warm chocolate mixture into the egg mixture. The warmth of the chocolate will help to temper the eggs, preventing them from scrambling.
With your chocolate and egg mixtures combined, gently stir in the all-purpose flour until just incorporated. Be careful not to overmix, as this can lead to tougher brownies.
Finally, fold in the chopped Reese’s peanut butter cups. This is where the magic happens! The pieces will melt slightly during baking, providing delightful pockets of peanut butter goodness.
Pour the brownie batter into the prepared baking pan, spreading it evenly. Use a rubber spatula to level the surface, ensuring even baking.
Place the pan in the preheated oven and bake for approximately 35 to 40 minutes. The brownies will puff slightly, and a toothpick inserted into the center should come out mostly clean.
Once baked, remove the brownies from the oven and allow them to cool in the pan for about 2 hours at room temperature. This cooling time is essential for achieving the perfect texture.
Once cooled, cut the brownies into 1-inch squares and serve. Store any leftovers in a tightly sealed container at room temperature for up to 3 days.