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 egg. Mix until fully incorporated.
Add the flour and mix for 2min on high speed.
Butter the skillets and fill half of the batter in each. Reserve.