Step 1

Preheat oven to 350 F/175 C. Line the bottom of a round 12 inch/30 cm baking pan with parchment paper. Set aside.

Step 2

In a large clean mixing bowl, add in 4 egg whites reserving the egg yolks in a separate bowl, and a pinch of salt, then begin to whip the egg whites on medium speed. Once the egg whites start to foam, begin adding in the sugar a little at a time until all is incorporated. Continue whipping until the egg whites are firm and double in size (about 4-5 minutes) on medium-high speed.

Step 3

Once the egg whites are firm, turn off the mixer and gently fold in ground walnuts and flour. Once folded in, gently transfer the batter to your lined baking pan and spread in an even layer. Bake for 15-20 minutes until lightly brown on top and set to the touch. Remove the pan from the oven and carefully remove the cake layer. Repeat this step 3 more times.

Step 4

In a heat-proof bowl, add in all the egg yolks, sugar, and vanilla sugar. Also, add in the vanilla extract at this point if using. Begin to whip the egg yolks and sugar on medium-high speed until thick and light in color (2-3 minutes).

Step 5

Place the bowl on top of a small pot filled with a cup or so of water. Make sure the water is not touching the bowl. This is the double broiler method. Turn the heat to medium and begin to whisk the egg yolk mixture while the water comes to a boil. Maintain a soft boil of the water and cook the egg mixture slowly for about 25-30 minutes whisking the entire time ensuring the eggs do not scorch. Add more water to the pot if need but never enough to touch the bowl.

Step 6

After about 25 minutes of cooking the egg yolks, you'll notice they have become considerably thicker. You are looking for a hot pudding-like consistency. Another way to tell if your mixture is ready is when you run your wooden spoon or whisk through it should leave a noticeable trail that closes in on itself slowly rather than immediately.

Step 7

When the mixture has thickened add in the chocolate, breaking it up with your hands. Whisk until the chocolate has melted. Remove the bowl from the pot, cover with plastic wrap (laying it carefully directly on top of the mixture), and allow to cool completely. If assembling the cake in a few hours, refrigerate the mixture until ready to use.

Step 8

When your chocolate mixture has cooled completely, whip your room temperature butter until fluffy, then add the chocolate egg yolk mixture a little at a time until both are incorporated. You will notice the finished filling has turned a light brown color. Set aside.

Step 9

Place the first layer of sponge cake on your cake stand. Spread it with a semi-thick layer of chocolate filling. Place the next cake layer on top and repeat the process until all your filling and cake layers are used up. Use any remaining chocolate filling if you have it to cover the sides of the cake.

Step 10

Prepare your whipped cream and cover the sides of the cake with it. Press ground walnuts to the sides of the cake.

Step 11

In a small pot, melt the remaining chocolate and oil together until smooth and melted. Pour the chocolate on top of the cake and spread in an even layer. Refrigerate briefly until the chocolate has set, then decorate the edges of the cake with additional whipped cream and/or ground walnuts.***Sometimes I like to cover the cake completely with whipped cream, refrigerate briefly, and then pour the chocolate glaze on top of the whipped cream (like the picture). It's slightly messier but makes the cake slightly taller.***