Separate the yolks and put them in a bowl. Add the sugar, cocoa, soft butter and beat well until you get a thick cream; set aside 1/3 of it to serve as the cake glaze.
Put the walnuts, vanilla and beaten to snow egg whites in the cream. Stir carefully with a wooden spoon until the mixture homogenizes, so the egg whites don't deflate.
Pour the mixture into a buttered round oven dish, laid out with baking paper, and bake for 1 hour 20 min. in a low 248°F (120 °C) oven.
While the cake layer is still warm, smear it with the set aside cream (decorate with walnuts if desired). Leave the cake to set in the fridge for 6 hours and then serve.
A very delicious, lovely and easy recipe. Bon appetit!