Preheat the oven to 450°F and place a rack in the center of your oven. In the bowl of an electric mixer, beat 10 egg yolks for a few minutes at high speed until pale and lemon-colored. Reduce speed and gradually add sugar, then increase the speed and beat the yolks and sugar until thick and glossy. Scrape bowl occasionally with rubber spatula. Reduce speed again and gradually add flour and salt; increase speed mix for 5 minutes more, then mix in lemon juice. Scrape bowl again with a rubber spatula. In a separate bowl with cleaned beaters, beat the 7 egg whites with a whisk attachment until they hold stiff peaks. Stir a few heaping spoonfuls of the whites into the yolk mixture to loosen the mixture, before folding in the rest of the whites in three additions until the batter is a spreadable, foamy consistency.

Spread the batter in prepared pans or within stenciled shapes on parchment paper. Bake each layer for 5 minutes, or until golden with some dark brown spots. When a layer is baked, remove it from the oven and flip it out onto a cooling rack dusted with a small amount of confectioners’ sugar. Carefully remove parchment paper then flip cake back onto another lightly dusted cooling rack to finish cooling. Repeat with remaining layers. Cool the layers right side up.

Melt chocolate until smooth. Set aside to cool to room temperature. In the bowl of an electric mixer, beat butter until soft and smooth, scraping frequently. Add vanilla and 3 egg yolks. Add sugar and cooled chocolate, beating until thoroughly mixed and scraping as needed.

Place four strips of parchment or waxed paper around the outer edges of your cake plate. Place the first cake layer on the plate and spread chocolate on top and to the edges with an offset spatula. Repeat with remaining layers (or all layers except one, if a decorative caramel layer is desired), stacking cake as evenly as possible. Once fully stacked and filled, trim the edges again so that they’re even.

Spread chocolate on the outside of the cake in a thin coat, just to cover and adhere the crumbs to the cake. Place the cake in the fridge for 30 minutes (or freezer for 5 minutes) to set the chocolate. Spread chocolate more thickly and smoothly to make a final exterior coat of frosting. Remove paper strips.

For the caramel topping (optional): Lightly grease a sheet of parchment paper. Place the last cake layer on this sheet. Lightly oil a large chef’s knife (if cutting layer into 16 traditional wedges) or sharp cookie cutter of your choice and set aside. Combine the sugar and water in a small, heavy saucepan and swirl it until the sugar melts and begins to turn a pale amber color. Quickly pour this over the prepared cake layer and spread it evenly with an offset spatula. Using the prepared knife or cutter, quickly cut the layer as desired. Leave in place, then cool completely. Once fully cooled, cut edges of shapes again to ensure they can be removed cleanly. Arrange caramel pieces or wedges over the cake, propping them up decoratively with hazelnuts. Chill cake until needed.