Preheat your oven to 180ºc/160ºc fan, and line a 9x9" square tin with parchment paper.
Melt the dark chocolate with the butter and leave to cool slightly
Whisk together the eggs and sugar until really thick and mousse like - it will double in volume, be a lot paler, and leave a trail for a couple of seconds before disappearing when done
Fold the chocolate/butter mix into the eggs/sugar mix carefully
Add in the plain flour and cocoa powder, and fold that in as well
Add in the chocolate chips and peanut butter cups and fold through
Pour the brownie mixture into the lined tin
Drizzle on the melted peanut butter and swirl in ever so slightly
Bake the brownies in the oven for 25-30 minutes, or until baked through
Leave the brownies to cool fully in the tin, and then set in the fridge for 3+ hours if you can for the best brownie
Notes
You can use smooth or crunchy peanut butter
You can replace/add
100g peanuts, chopped
100g peanut butter chips
Any flavour of chocolate chip
These will last for 4-5+ days at room temp or in the fridge
These will freeze for 3+ months
I use this 9" square tin
I recommend this oven thermometer to make sure your oven is correct
Bake until there is a small wobble in the middle of the tin
Set the brownies in the fridge until solid once baked and cooled