Preheat the oven to 350°F (175°C). Line baking sheets with parchment paper.
Beat the egg whites with the salt until foamy. Gradually add the sugar and beat until stiff, glossy peaks form.
Gently fold in the ground hazelnuts, flour, and vanilla.
Divide the batter into 5–6 thin equal circles, about 8 inches (20 cm) wide.
Bake each layer for 7–10 minutes, until lightly golden. Cool completely.
For the chocolate cream, whisk the egg yolks, sugar, cornstarch, and milk in a saucepan.
Cook over medium-low heat, stirring constantly, until thickened. Remove from the heat and stir in the vanilla. Cool completely.
Beat the softened butter until fluffy. Gradually add the cooled custard, then beat in the melted chocolate.
Place one cake layer on a serving plate and spread with chocolate cream. Repeat with the remaining layers, finishing with the top layer.
Warm the apricot jam and spread a very thin layer over the top.
Mix the powdered sugar with lemon juice until smooth. Spread evenly over the top of the cake.
Immediately pipe thin lines of melted chocolate across the icing. Drag a toothpick through the lines in alternating directions to create the classic feathered pattern.
Press chopped hazelnuts around the sides of the cake.
Refrigerate for at least 4 hours, preferably overnight, before slicing.
