In a deep pan over medium heat, melt the couverture and butter, stirring with the cocoa powder and hot water until smooth.
Remove from the heat and add the sugar, egg yolks, flour, and salt.
In a mixer, beat the 4 egg whites to a meringue.
Add the meringue to the original mixture and mix gently from the bottom up until combined.
Pour the mixture into a 24cm mold after placing the parchment paper.
Bake at 180°C for 40 minutes.
The same cake can be made without flour if there is a dietary restriction.