Preheat your oven to 350 degrees Fahrenheit and prepare a square baking pan (I use an 8-inch square paby greasing it with some butter.
In a medium bowl, combine the melted butter, Reese spread, brown sugar, unsweetened cocoa powder, egg and vanilla with a wire whisk until well combined.
Add the flour and baking powder and fold it into the wet ingredients with a rubber spatula just until no streaks of flour remain.
Scrape the batter into the prepared baking pan and smooth out the top as well as possible.
Bake the brownies for about 20-25 minutes at 350 degrees Fahrenheit until they appear set in the middle.
Let the brownies cool in the pan for about 10 minutes.
After they've cooled for about 10 minutes, dollop the Reese spread and the all natural peanut butter on top of the brownies and use a knife or wooden skewer to carefully marble the two toppings together to create a swirl pattern. Since the brownies will still be warm the peanut butter and Reese spread will begin to melt slightly, which will make the swirling easier.
Let the brownies sit at room temperature to cool completely before cutting to serve. Alternatively, place the brownies in the fridge for an hour to set, and then remove to room temperature for about 15 minutes before serving.
The result will be an ultra-chewy brownie with a hint of peanut butter flavour and a super sweet-and-salty topping that will satisfy any peanut butter chocolate craving!