Preheat oven to 350F with the rack centered. Line a 10-inch round springform or baking pan with aluminum foil or parchment paper (for easier removal and cleanup) and set aside. Gather all the ingredients needed for the cake. 

In a bowl of a mixer fitted with a whip attachment combine the egg whites, sugar and salt. Whip for about 15-20 minutes or until very shiny, and stiff peaks form when the whisk is lifted up from the meringue. 

Transfer to a piping bag fitted with a large round tip. Or just use a gallon sized ziplock with a snipped end. Set aside. Alternatively, you can just spoon the meringue.

Make the dough: Combine the egg yolks, sugar, salt, and vanilla in a large bowl and whisk until smooth. Add sour cream and smooth with a whisk again. Next, add the butter and whisk until combined. Now add the flour, baking soda, and baking powder and mix until dough forms. Divide the dough into 2 or three equal pieces (depending on how many layers you prefer).See bottom of recipe card for notes.

Assemble the cake layers: Spread 1 piece of dough per pan, into an even layer. Top with 1/3 of the plum butter, then pipe or spoon 1/3 of the prepared meringue on top keeping 1/2 an inch away from the sides of the baking pan as the meringue will expand while baking.

Now sprinkle 1/3 of the chopped walnuts.

Repeat the above steps 2 more times, to make 3 cake layers total.

Bake the Cake Layers: Bake the prepared cake layers for about 40 minutes. Check the oven at about 25-30 minutes, if the meringue is browning too quickly, tent the baking pans with a flat piece of parchment paper and continue baking for another 15 minutes. 

Remove from the oven, allow to cool in the pan for about 10 minutes, then remove the cake layers with the removable bottom and allow to cool completely. Be gentle as the cake layers are very fragile and crack easy. 

Heat the milk in the microwave or in a medium saucepan until boiling. 

Meanwhile, mix egg yolks, sugar, flour and a couple tablespoons of milk in a bowl until smooth. Now, add hot milk into the egg yolk mixture 1/2 a cup at a time, while whisking continuously. 

Pour the mixture back into the sauce pan and bring to a boil over medium heat stirring constantly. Cook 2-3 minutes past boiling. Remove from heat and put through a fine-mesh sieve to remove any clumps. Add vanilla and stir. 

Allow the custard to completely cool - to speed up this process, spread the custard in a large, rimmed baking sheet and cover with plastic wrap, touching the custard to prevent a film from forming.

In a mixer bowl whip 4 oz of butter until fluffy and pale in color. Add the custard in 3-4 additions, whipping after each. 
Transfer the custard to a piping bag fitted with a large round tip. Refrigerate until ready to use.

Combine the boiling milk with chocolate chips. Allow to sit for 1 minute. Stir until smooth. Set aside. 

Put a dab of frosting onto the serving platter or board. Carefully slide the first cake layer off the bottom of the springform on top and press to adhere. 
Pipe the Custard filling in dollops all over the top of the cake, staying about 1/3 inch away from the borders of the cake. 

Top with the second cake layer. 

Sprinkle the berries on top. 

Drizzle with the ganache. 

Refrigerate the cake for at least 8 hours for the cake to be stable.