Preheat oven to 350 degrees F. Line 9x13-inch baking sheet with foil or parchment paper, allowing excess to hang over the edges of the pan. Coat with non-stock cooking spray and set side.
Whisk together flour, cocoa powder, baking powder, and salt. Set aside.
In a large bowl, beat together sugar and melted butter. Once smooth, add eggs and vanilla. Mix until well combined.
Slowly add dry flour mixture, about 1/2 cup at time. Mix well until there are no longer any dry streaks of flour. Batter will be very thick. Spread batter in an even layer into prepared baking pan.
Place unwrapped caramels and milk in a microwave safe bowl. Heat at half power at 30 second intervals for 2-3 minutes. Stir after every interval. Once smooth, stir in 1/2 cup of peanuts. Pour over chocolate batter. Spread into an even layer.
Bake for 45 minutes. Remove from oven and immediately sprinkle chocolate pieces over brownies. Allow chocolate to stand for 2 minutes. Using a mini offset spatula (or the back of spoon), spread melted chocolate into an even layer. Distribute remaining 1/2 cup peanuts over chocolate layer.
To set chocolate layer, chill brownies in the fridge for 30 minutes. Lift brownies out of pan using the hangover foil (or parchment paper). Cut into desired size pieces.