Preheat oven to 350ºF.
Prepare a 8x8-inch pan by lining it with parchment paper and cooking spray. Apply cooking spray to bottom and sides of pan before adding parchment paper. Also, leave an overhang of parchment paper so you can use it to easily remove brownies from pan. If needed cross the parchment in the pan, vertically and horizontally so pan is totally covered.
In a large mixing bowl, add sugar, flour, butter, eggs, cocoa powder, vanilla, baking powder, and salt. Beat well until batter is smooth and scrape dowl the bowl as needed..
Transfer half of the batter into your prepared pan.
Sprinkle one bag of the Reese’s cups and candy over top or 1 cup of each.
Pour remaining batter on top of Reese's.
Sprinkle remaining bag of Reese’s evenly overtop of batter or remaining cup of each. Gently press them into batter.
Bake for 35 minutes on middle rack or until a toothpick comes out mostly clean with a few crumbs attached..
Allow brownies to cool in the pan for between 10 and 15 minutes then lift out of pan and place on rack with parchment to cool completely.
Once totally cool cut and serve.