Beat the egg yolks with the sugar until whitened. Add the milk in which you've beaten the cocoa. Put this mixture to boil on low heat until thickened, while stirring nonstop to prevent burning.
Once thickened, remove from the heat and leave it to cool.
Dissolve the gelatin in 2 tbsp cool water in a water bath. Add it to the cooled cake mixture, stirring well.
Finally, add the beaten to stiff snow egg whites. Stir everything well and pour the mixture into forms. Leave them to harden for a few hours.