Heat oven to 350°. Prepare a 9x13 baking dish by spraying with cooking spray, lining with parchment paper, or tin foil (spray the tin foil lightly if using).
In a large mixing bowl, mix together the melted butter and sugar with a wire whisk until combined. Add eggs, one at a time, mixing the egg into the batter with a whisk before adding the next egg.
Add vanilla extract and cocoa powder. Stir to combine. Add the flour and stir together until no flour streaks remain.
Spread out half the brownie batter in the bottom of the prepared pan. It is easier to spread out if you dollop spoonfuls of batter all over the pan instead of dumping into one pile in the middle.
Press the 20 Oreo cookies into the batter. Cover with the other half of the brownie batter. Again, drop spoonful's of batter over top the cookies (like in the pictures in the post). It also helps to spray the back of a stainless steel dinner spoon with cooking spray, and then use that to help spread out the batter.
Cook for 23-30 minutes. Mine are always done around 23 minutes so check yours if your oven cooks fast like mine.
The brownies will look puffy, slightly pulled away from the edges of pan, and a toothpick inserted in the middle should come out clean.
Immediately after taking the brownies out of the oven, lay the 8 Hershey candy bars over top the brownies. Let sit for about 15 minutes and then spread out the chocolate with a spoon. Let the brownies cool completely. The chocolate frosting will harden and set. This takes about 1-2 hours.
Store leftovers, covered, at room temperature. The frosting will stay hardened even at room temperature.