I found the recipe here http://www.madbanditten.dk/2011/03/06/verdens-bedste-chokoladekage-i-lchf-stil-hvis-man-vil/
150g dark chocolate (I used 85%)
150 g sugar *
150 g butter
150 g of egg yolk
150 g of egg white
* Or the equivalent of your favorite sweetener - I used sukrin
wisk the egg whites until fluffy and stiff.
Chocolate, butter and sugar are melted in a water bath ( when you have a bowl on top of a pot of boiling water). When chocolate mixture is consistent, stir the egg yolks in. I used pasteurized yolks for consistency and ease. Finally mix the stiff egg whites into the mass, and then you stir until you again have a smooth paste, then pour the whole lot into a greased pan and bake 20 minutes at 180 degrees/350 Fahrenheit. The cake can raise up and get a little weird shape, but it falls flat when it cools down.