Melt egg, sugar, butter, baking soda, honey, and milk gently over low heat until the mixture changes color.
Toast walnuts, grind finely, and combine with flour and cocoa. Stir into the warm base and knead into dough.
Roll dough into equal thin layers. Bake quickly on a floured baking sheet for a few minutes per layer.
For custard, whisk egg, sugar, flour, and milk. Cook gently until thickened; cool. Whip butter, then beat in cooled custard gradually.
Layer the cake with cream and chill for a couple of hours.
Melt chocolate and butter together and pour over the cake. Chill until set before slicing.