Beat the eggs until you get a fluffy cream, then add the ground walnuts, flour and baking powder.
Pour the mixture into an oiled baking dish, covered with baking paper. Bake in a preheated 338°F (170 °C) oven for 30-40 min. Then take out the ready cake layers.
Once they're cooled, start preparing the buttercream by first beating the confectionery cream.
Beat the butter (at room temperature) in another bowl, along with the powdered sugar, yolk. Gradually add the cream to them until you obtain a smooth mixture.
Stack the cake by arranging a cake layer, cream etc. Finish off by smearing the entire cake with the remaining cream.
Decorate with ground or whole walnuts if desired.