Beat the eggs, sugar and butter.
To them, add the flour. Set aside a part of it, to it add the cocoa.
Combine the 2 mixtures carefully, while stirring slowly to obtain a marble-like pattern.
Pour it into a buttered oven dish and bake the layer in a preheated 392°F (200 °C) oven for 30 min.
Put the milk with the sugar and vanilla on the stove to boil.
Set aside 4/5 cup (200 ml) of this mixture and add 4 tbsp flour and the citric acid to it. Put it on the stove and beat until it boils. Once it cools a bit, add in the butter.
Pour the remaining hot milk mixture over the baked and cooled layer.
Smear with the cream on top and sprinkle generously with ground walnuts.
Leave it to cool in the fridge before serving.