Chocolate mousse

Melted chocolate is fine cocoa particles, a dry powder, suspended in fat, the cocoa butter. The water in the yolks wets those particles, they stick to one another and push the cocoa butter out: the paste seizes.

Nestlé Dessert holds together for three reasons: at 53%, it has half as many cocoa particles as a 90%; half its mass is sugar, which dissolves in the water instead of clumping in it; and it contains lecithin, an emulsifier that coats the grains and makes them slide rather than catch. It is a chocolate built to be melted and stirred.

Neither Dardenne 100% nor Ethiquable 88% contains any lecithin, and both hold far less sugar: four times less for the Ethiquable, almost none for the Dardenne. Hence the quarter of the whites in step 5.