- Preheat the oven to 350℉/180℃. Grease an 8×8 inch square pan. For easy removal of the brownies, place parchment paper on the bottom and 2 sides of the pan. Grease again and then set side. 
- In a large bowl, add melted butter and white sugar until well combined. It will look glossy, but have grains which is okay. - 1/2 cup butter, 1  cup white sugar 
- Add eggs and vanilla and continue whisking. - 2 eggs, 1 tbsp vanilla 
- Toss in flour, salt, and cocoa powder. Using a spatula, mix until a brownie batter forms. Don't overmix! - 3/4 cups all-purpose flour, 3/4 cups cocoa powder, 1/2 tsp salt 
- Add in the Reese's cups and mix in. - 1/2 cup Reese's peanut butter cups 
- Transfer into the prepared pan and spread evenly. Sprinkle extra peanut butter cups if desired. 
- Bake the brownies for 30-35 minutes. A toothpick inserted should come out fudgy. If it is wet, it isn't ready yet. Continue to bake at 5 minute intervals until fudgy in the middle, and the edges are set. 
- The brownies continue to bake as they cool in the pan. Allow them to sit until cooled. Slice, and enjoy! 
- Store the brownies on your counter for 3 days or in the fridge in an airtight container for 7.