Preheat the oven to 350°F / 180°C. Butter and line 2 x 4.5 inch springform pans with parchment paper. Make sure the parchment paper rises 1-2 inches above the top rim of the pan (I also prepared an extra ½ cup ramekin by buttering it for the extra batter that might be left over).
Place the chocolate, salt, 2 tbsp balsamic vinegar, and butter (or oil) and melt the chocolate in the microwave in 20-30 second bursts, stirring in between. Alternatively, you can melt the chocolate over a pot of simmering water (in a double boiler), making sure that the hot water does not make contact with the bowl or the chocolate.
Leave the melted chocolate to cool, until you prepare the eggs.
In a large bowl, start whisking the egg white with a hand beater until frothy. Turn the speed to high and gradually add 2 oz of the sugar while whisking. Whisk the egg whites till they have tripled in volume, glossy, thick, and have stiff peaks. Set aside.
Add the suagr, coffee / vanilla to the egg yolks and whisk until the mix turns pale, thick and reaches the ribbon stage.
Add the melted and cooled chocolate mix to the egg yolks. Mix through.
Add ⅓ of the whisked egg whites to the chocolate mix and fold through to loosen the mix (Selma recommends using a big metal spoon to avoid deflation of the egg white mix).
Add the rest of the egg whites in 2 batches, folding them in gently until well mixed. Be very careful not to deflate the mixture.
Pour the mixtures into the prepared springform pans. Pour the batter up to the top rim of the pan, and stud the cake with the raspberries. Pour any extra batter into the ramekin (this can be your “tester”).
Bake in the preheated oven for 15-20 minutes until the cake rises and an inserted toothpick comes out with a few crumbs attached to it.
The cake will have risen when it comes out of the oven, but will deflate as it cools down.
Serve at room temperature or after chilled in the fridge with extra berries, a dusting of icing sugar and optionally with some whipped cream (or a dairy free substitute).