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.

Add the flour and mix for 2min on high speed.

Line the mould with baking paper and take to a preheated oven at 160C for 20-30min or until you can insert a toothpick at the center and it comes out with a bit of dough (we don't want it to dry out completely).

Let cool and cut.