For the sheets, mix the egg whites with a pinch of salt until they become foam. Then add the sugar and mix until the foam becomes shiny.
Mix the flour with the ground walnuts and baking powder and gradually add it to the egg foam. Stir gently, moving from bottom to top until incorporated. At the end we add the vanilla extract.
Divide the composition into three equal parts. We will bake three sheets in a round tray (26cm), lined with baking paper, over low heat until golden (~ 15 minutes).
Remove the sheets from the tray and leave them to cool, without overlapping them. When they have cooled completely, remove the baking paper.
Meanwhile, prepare the cream. We hydrate the gelatin in 3 tablespoons of cold water. Separately mix the yolks with the sugar until they turn white and double in volume. Then add the cold milk and chopped chocolate. Put the pot to boil on a steam bath and mix until the chocolate melts and the mixture thickens.
Then remove from the heat and let it cool until it reaches a bearable temperature on the finger, then add the mascarpone cheese and mix until it becomes a fine cream.
Dissolve the gelatin over low heat, taking care not to boil. Remove from the heat and add a tablespoon of cream to reach the same temperature. Then add the gelatin over the cream and mix well.
Whip the whipping cream and then lightly incorporate it into the cream.
On a plate we place a sheet with walnuts and mount the cake ring around it. Put half of the cream on the sheet and level. Then the second sheet and the rest of the cream. Cover with the last sheet and put the cake in the fridge.
For the caramel, melt the sugar until it turns golden, then add the butter and mix vigorously until it melts. At the end, add the whipping cream and let it boil until it thickens. Remove from the heat and let cool. When it has reached room temperature, spread the caramel on the cake and put it in the fridge
for a few hours or preferably overnight.
To garnish, melt the sugar until golden, then add the whole walnut kernel and mix well. Pour the composition on baking paper and let it cool. We then put the caramelized walnuts in a bag and crush them with the rolling pin or we can crush them in the food processor.
Take the cake out of the fridge and remove the ring. Grease the edge with a thin layer of whipped cream and then garnish with caramelized walnuts.
From the remaining whipped cream we can decorate according to preferences.