Preheat the oven to 180°C. Line an 20x20cm pan with baking paper or aluminum foil, leaving a 2.5cm overhang on all sides. Spread oil or butter on top and set aside.
Melt the chocolate together with the butter in a heatproof bowl over a pot of simmering water. Stir constantly until the mixture is smooth.
When melted add the vanilla and plum schnaps, let it cool down a bit.
In a separate bowl mix together the egg yolks with the sugar and add to the cooled chocolate.
Beat the egg whites with the pinch of salt and mix it gently until it looks shiny.
Spread the mixture evenly into the baking tray and bake for 35 minutes. During that time do not open the oven door!
To check if the brownie is ready, insert a toothpick into the tray center. If it comes out with a few moist crumbs clinging to it - its ready.
Lastly, let cool for at least 1 hour.