Position the rack in the center of the oven and preheat to 180°C (355°F).
Lightly butter a 430 mm x 280 mm (17″ x 11″ ) baking pan. Line the bottom and the sides of the pan with parchment paper. Make a diagonal slice in each paper corner with a cut extending to the corner of the pan. This will allow to fold paper corners neatly. Trim the excess paper sticking above the pan sides.
In a small bowl mix together 5 tbsp of ground walnuts and 4 tbsp of flour, set aside.
In a large bowl, beat 6 egg whites with 1/4 tsp of cream of tartar until soft peaks form. Gradually add 2 tbsp of granulated sugar and beat until stiff, shiny peaks form.
In a large bowl, on high speed, beat 4 egg yolks with 3 tbsp of granulated sugar until fluffy, pale, and yellow (5 minutes).
Add half of the walnut/flour mixture to the egg mixture and using a large balloon whisk, gently fold it in. Add the remaining walnut/flour mixture and fold it in.
Add about 1/4 of egg whites to egg yolk mixture and mix thoroughly until thinned up.
Add the remaining egg whites to the egg yolk mixture. Using silicone spatula, carefully fold egg whites into thinned egg yolk mixture. Use a scooping motion from the middle towards the sides of the bowl. As you come to the side of the bowl rotate the wrist and let the mixture fold onto itself. Repeat and each time rotate the mixing bowl slightly. Also scoop the perimeter of the bowl with the spatula. You want to minimize folding time to avoid deflating the egg whites.
Use an offset metal spatula to spread the nut batter evenly into the baking pan.
Bake until golden brown, about 15 minutes. Let cool completely.
Repeat the whole process once again and bake another cake layer.
Buttercream Filling
In a large bowl, on high speed, beat 12 egg yolks with 175 g of granulated sugar until pale and yellow (2-3 minutes). Mix in 4 tbsp of corn starch.
Over medium heat, bring 1L of milk to boil, stir frequently to preventing burning at the bottom of the sauce pan.
Slowly pour about 250 mL of hot milk to egg yolk mixture to temper it, whisking constantly.
Slowly add tempered egg yolk mixture to the milk and keep whisking constantly. Cook until the mixture thickened up. Whisk constantly to prevent lumps.
Cool off in a water bath, whisk constantly to prevent skin from forming (10 -15 minutes). Bring down to room temperature.
Place plastic film pressing directly to the surface of the cooled egg yolk mixture. This will prevent a skin from forming.
In a medium size mixing bowl, mix together 250 g of softened butter and 50 g granulated sugar, at high speed until soft and fluffy, about 5 minutes.
Reduce speed to low and mix in cooled egg yolk mixture, one spoon at a time.
Using a microplane grater, grate 1/2 of whole nutmeg, mix it in.
Gently fold in first chopped walnuts and then ground walnuts, set aside.
Assembly
Cut each baked cake layer in two 20 cm x 28 cm (8″x 11″) halves keeping the parchment paper attached to the cake layers.
Place the first cake layer faced down on a rectangular serving plate with parchment paper on the top. Peel off the parchment paper.
Using an offset spatula, evenly spread approximately 1/4 of buttercream on top of the cake layer.
Place the second cake layer faced down on top of the buttercream. Peel off the parchment pepper. Using an offset spatula, evenly spread approximately 1/4 of buttercream on top of the cake layer.
Repeat the process with the other 2 cake layers.
Add the buttercream all around the cake sides to even them out.
Using an offset spatula add ground walnuts to the cake sides. Sprinkle ground walnuts on top of the cake.
Add 10 walnut halves evenly spaced along 2 longer sides of the cake.