Prepare a medium bowl greased with butter or oil (we need it greased so that caramelized walnuts don't stick to the bowl).

Add sugar in a large heavy bottom pan and let it melt on medium-low heat, stirring occasionally.

When sugar is melted and starts to get a dark golden color, add walnuts and with a wooden spoon mix everything so that walnuts are coated in melted sugar.

Continue cooking and mixing for few minutes until sugar is a dark amber color and start to smell like caramel.

Carefully transfer to a prepared greased bowl and let it cool completely.

Place sugar and water into a medium saucepan and stir to combine. Cook over medium-high heat and stir constantly until the sugar has dissolved and the mixture starts to bubble just a little. Increase heat and bring to a boil, do not stir anymore.

Let the mixture continue boiling until it turns an amber color. Remove from the heat and slowly whisk in the heavy cream. The mixture will bubble up quite a bit and boil. Then add butter and vanilla, return to heat and constantly mixing bring to a boil. Cook 1 minute, whisking constantly. Remove from heat and let cool to room temperature. Transfer to a container and place it in the fridge to thicken.

Preheat oven to 180 C (350 F).

Prepare 3-20 cm (8-inch) springform pans, bottom lined with parchment paper, sides greased and floured.

Separate egg yolks and egg whites.

In a bowl, whisk flour, ground walnuts, and baking powder. With an electric mixer beat egg whites with salt until stiff peaks form. In a separate bowl beat egg yolks and sugar until pale and frothy. Carefully fold in egg yolks mixture into beaten egg whites. Add dry ingredients and using a rubber spatula gently mix until everything is incorporated. Dived batter between 3-20 cm (8-inch) springform pans and bake 10-15 minutes (I baked mine 13 minutes).

There are two options and both work well, only the texture is different.

Divide cooled caramelized walnuts in half.

The first option is to finely ground one-half of caramelized walnuts and finely chop the second half (will use this half for sprinkling and decoration) - this is how my mom makes the filling.

What I like to do, to get more smoother filling is to put the first half of caramelized walnuts into a food processor and process until the buttery texture is formed (it won't be completely smooth, there will be still bits of caramelized sugar and that's ok) and finely chop the 2nd half. It's up to you.

Next, pour milk (leave about 5 tablespoons) in a large saucepan and on medium heat bring to a light boil.

In the meantime rest of the milk (5 Tbsp) mix with vanilla pudding powder, sugar, cornstarch, and vanilla extract.

Pour mixture into hot milk and cook, stirring constantly, about 3-4 minutes until thick.

Leave it to cool. Beat butter until creamy and smooth and mix into cooled pudding. Add in caramelized walnuts (finely ground or in the buttery form) and mix everything until fully incorporated. Divide the filling into 3 parts.

Put one cake layer on the cake stand or platter.

Spread one part of filling and cover with the second cake layer.

Than again filing and third cake layer. With the rest of the filling cover entire cake, top and sides.

If you decided to use caramel (if caramel has thickened too much, microwave in 5-10 seconds intervals until it's pourable (but not warm)) spread it over the top and using a teaspoon make drips on the sides of the cake, sprinkle the top with chopped caramelized walnuts, or if you decided to omit caramel, sprinkle the whole cake, top and sides with chopped caramel walnuts (this is how my mom do it).

Refrigerate for at least 2 hours before serving and slicing.