Prepare the Chocolate Mixture: (WATCH VIDEO BELOW) In a small saucepan, melt ½ cup of unsalted butter over low heat. Add 1½ cups of semi-sweet chocolate chips and stir continuously until completely melted and smooth. Remove from heat and set aside to cool slightly — you want it warm but not steaming when it hits the eggs.
Mix the Sugars: In a large mixing bowl, combine ½ cup of white sugar and ½ cup of packed brown sugar. Whisk until well combined.
Incorporate the Wet Ingredients: Add 3 large eggs, 1 teaspoon of vanilla extract, and 1 teaspoon of salt to the sugar mixture. Whisk until smooth and slightly fluffy. Pour in the cooled chocolate-butter mixture and whisk again until fully combined and glossy.
Combine the Dry Ingredients: Gradually sift in ¾ cup of all-purpose flour, ½ cup of unsweetened cocoa powder, and ¼ teaspoon of baking powder. Gently fold with a spatula until the batter is smooth and free of lumps — don't overmix. Stop as soon as you don't see dry streaks.
Fold in the Reese's: (WATCH VIDEO BELOW) Add 1½ cups of roughly chopped Reese's cups to the batter. Keep the pieces chunky — you want those melty pockets of peanut butter throughout the brownie. Fold gently until evenly distributed.
Preheat and Prepare: Preheat your oven to 350°F (175°C). Line a 9"x9" baking dish with parchment paper and set aside. An 8"x8" works too — your brownies will just come out slightly thicker and may need an extra 2–3 minutes in the oven.
Pour and Top: Pour the batter into the prepared baking dish and spread it evenly with a spatula. Press your toppings on — halved Reese's cups, mini cups, Reese's Pieces, or whatever combination you're going with. Press them gently into the surface so they bake in.
Bake: Bake for about 25 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs for fudgier brownies, or clean for cakier ones. Start checking at 23 minutes — the peanut butter in the Reese's can make the edges brown a little faster than a plain brownie batter would, so keep an eye on them.
Cool and Slice: Allow the brownies to cool in the baking dish for at least 20 minutes before slicing. This step is actually important — cutting too early means crumbly edges and a messy slice. Let them set up fully for clean squares.
