Preheat oven to 350 degrees.
Grease a cast-iron skillet with softened butter, cooking spray, or neutral oil
In the microwave, melt butter and chocolate in a bowl at 20 second intervals until completely melted and smooth.
Sift cocoa powder over the melted chocolate mixture. Whisk until smooth and well-combined. Set aside to cool slightly.
In the bowl of a stand mixer, mix together eggs and sugar until pale and creamy.
Add in the melted chocolate mixture and gently fold together using a rubber spatula.
Gently fold in flour.
Pour batter into the prepared skillet, using a rubber spatula to evenly spread to the edges.
Use a tablespoon to dollop dots of peanut butter over top of brownie batter. Then take a butter knife and swirl the peanut butter into the batter.
Bake for 25-30 minutes or until the edges have set. Test with a toothpick or knife to ensure they are done.
Allow to cool for at least 10 minutes then serve!! I suggest using a plastic knife to cut the brownies, as this prevents them from breaking. Enjoy!