filling must always be freshly made

From 1922:

Take 6 oz of freshly-peeled walnuts, blanch them and dry well. Reserve 2 oz. Have 4 oz of freshly-blanched almonds also. Chop the nuts roughly and add to them 6 oz of sifted sugar and the yolks of 6 eggs very well whisked. Mix in by degrees, whisking all the time, 4 oz of finely sifted flour into which has been mixed a teaspoonful of baking powder. Lastly, add quickly the stiffly-whisked whites of the eggs, pinch of salt and a few drops of lemon juice. Place in a well-protected cake tin and bake three-quarters of an hour in a moderate oven.
Decorate with icing, flavored slightly with lemon and ornamented with shelled walnuts. Should you wish to have this cake as a layer cake, cut into three rounds and spread with a filling made from icing sugar and crushed walnuts worked to a paste with a little thick cream. This filling must always be freshly made as it does not keep well.