Beat the egg whites with the butter and sugar until frothy. Mix the chopped nuts with the corn starch and fold the mass into the egg white mixture. Form 4 equal cake bases from the mixture and bake them for 120 minutes at 250°F. Once they're done baking, allow the cakes to chill.
Whip the butter until frothy, stir in the chocolate hazelnut spread, fold in the cocoa powder, and refrigerate for 30 minutes.
Beat the eggs, sugar, and vanilla extract until frothy and then stir in the corn starch and milk. Pour the mixture into a saucepan and bring to a boil over low temperature, stirring regularly, until the mixture thickens a little and then let it cool down.
Toast the sliced almonds in a pan and let them cool. Bring the sugar and water to a boil and cook until golden brown. Pour the liquid caramel on a piece of parchment paper and let it cool slightly before sprinkling the chopped nuts on top.
Spread one of the cake bases with chocolate cream and then put a second cake base on top. Spread vanilla cream over the second cake base and put the third cake base on top. Spread chocolate cream over the third cake base, top with the final cake base, and then coat the entire cake with the rest of the chocolate cream. Garnish the cake with the toasted sliced almonds, caramel brittle, and whole nuts.