Place the butter in cubes, the sugar and chocolate in a bain marie. Use a bowl where you're going to be beating the batter later. Make sure that the bottom of the bowl doesn't touch the surface of the bain-marie's water.
Once everything is melted, mix and remove from the heat. Wait for it to cook down a bit, about 10 minutes. We want it to still be hot, but not burning hot
After this time, add the vanilla, one of the eggs and beat until it's completely incorporated. Then add the other egg and beat on high speed for 2 minutes. If you're mixing it by hand, you want to mix about 75 times.
Add the flour and salt and beat for another 2 minutes on high speed. It's these beating intervals that give us that papery texture on top.