Put the butter, sugar, cacao, salt and vanilla essence in the bowl where you're going to whip them up later.
Put the bowl on a bain-marie without touching the surface of the water until everything is melted and mixed together.
Remove the bowl from the bain marie and wait 5min for it to cool down a bit. You should be able to touch the outside of the bowl without burning yourself.
Mix using a stand or hand-held mixer and add the eggs one at a time. Mix until fully incorporated + 3 minutes.
Add the flour and mix for 3 minutes on high speed.
Put the mix into a mould lined with baking paper and flatten it out.
Mix the peanut butter with the sugar until smooth. Put teaspoon-sized dollops of it scattered around the brownie and make swirls using a chopstick.
Take to a preheated oven at 160°C/320℉ for 20-30min or until you can insert a toothpick at the centre and it comes out with a few wet crumbs (we don't want it to dry out completely).
Let cool and cut.