Preheat the Oven

1. Preheat your oven to 350°F (175°C).  

Prepare the Layers

2. First Layer:
   • Separate the egg whites from the yolks, setting the yolks aside for the filling.
   • Place the egg whites in a clean mixing bowl and start mixing at a medium speed.
   • Add a pinch of salt to the egg whites. Continue mixing until they become frothy.
   • Gradually add sugar, one tablespoon at a time, while continuing to mix. Beat the mixture until stiff peaks form.
   • Gently fold in the flour and ground walnuts, being careful not to deflate the egg whites.
   • Prepare a cookie sheet by either spraying it with non-stick spray or lining it with parchment paper.
   • Spread the egg white mixture evenly on the cookie sheet, aiming for a thin layer.
   • Bake in the preheated oven for 12 to 15 minutes, or until lightly golden and set.
   • Remove from the oven and allow the layer to cool completely before gently peeling it off the parchment paper.
   • Set this layer aside and repeat the process three more times to create four separate layers. Place parchment paper between each layer to prevent sticking while you prepare the filling.

Prepare the Filling

3. Egg Yolk Mixture:
   • Add the reserved egg yolks to a clean mixing bowl with the granulated sugar.
   • Mix on medium-high speed for several minutes until the mixture becomes lighter in color and doubles in volume.
   • Prepare a double boiler by adding 2 to 3 cups of water to a pot and bringing it to a low simmer.
   • Transfer the egg yolk mixture to a stainless steel bowl and place it over the simmering water, making sure the bottom of the bowl does not touch the water.
   • Cook the mixture, stirring constantly, for about 10 minutes or until it thickens to a custard-like consistency.
   • Remove from heat and allow the mixture to cool completely.
4. Butter Mixture:
   • While the egg yolk mixture is cooling, clean the mixing bowl and add the softened butter.
   • Add 1 cup of powdered sugar (adjust to taste if you prefer less sweetness) to the butter.
   • Beat the butter and sugar together until light and fluffy, and the mixture has doubled in size.
   • Once the egg yolk mixture has cooled, gradually add it to the butter mixture, mixing until smooth and well combined.
   • Divide the filling into four equal portions.

Assemble the Cake

5. Layering:
   • Place the first layer on your serving plate.
   • Spread one-fourth of the filling evenly over the layer.
   • Place the second layer on top, and repeat the process, continuing until all four layers are stacked with filling between them.
   • Spread the final portion of filling on the top layer, ensuring it’s evenly coated.

Prepare the Topping

6. Caramelized Walnuts:
   • In a small pan, melt sugar over medium heat until it turns a light brown caramel color. Be careful not to let it burn.
   • Add the chopped walnuts to the caramelized sugar and stir to coat evenly.
   • Quickly pour the mixture onto a piece of parchment paper and spread it as thin as possible using a spatula. Be cautious, as the caramel is extremely hot.
   • Allow the caramelized walnut mixture to cool and harden completely.
   • Once hardened, crush it into small pieces and sprinkle it generously over the top layer of the cake.

Final Steps

7. Serving:
   • For the best flavor, allow the cake to set in the refrigerator for at least a few hours or overnight before serving.
   • Slice and serve, enjoying the layers of delicate cake and rich filling with the crunch of caramelized walnuts on top.