Heat the oven to 375 F. Line the bottoms of the cake pans with parchment and prepare six clean parchment rounds for the layers.
Beat the egg yolks with 1 cup sugar and vanilla until thick. In a clean bowl, beat the whites with salt, gradually adding the remaining 1/2 cup sugar to soft glossy peaks.
Fold the whites into the yolks in three additions, then fold in the flour and melted butter. Spread one-sixth of the batter in a prepared pan or on a marked parchment round.
Bake each thin layer for 7 to 9 minutes until pale golden. Cool flat and peel away the parchment. Repeat to make six layers.
Make the Buttercream:
Whisk the eggs and sugar in a heatproof bowl over barely simmering water until the mixture reaches 160 F and thickens. Beat off heat until cool.
Melt the chocolate and cool until barely warm. Beat the butter until smooth, then gradually beat in the egg mixture, chocolate, cocoa, and vanilla.
Assemble and Caramelize:
Reserve the neatest sponge layer for the top. Sandwich the other five layers with thin, even buttercream, then coat the sides and chill for 30 minutes.
Place the reserved layer on a buttered surface. Cook the sugar, water, and lemon juice without stirring until deep amber, then pour it over the layer and spread quickly.
While the caramel is still pliable, use a buttered knife to mark and cut 12 wedges. Cool completely, then arrange the wedges in a slight fan over the cake.
Serve:
Chill the assembled torte for another 30 minutes. Cut with a warm serrated knife between the caramel wedges and serve at cool room temperature.
Notes
Bake the thin sponge layers in batches; keep the batter covered and work efficiently.
Caramel hardens quickly, so butter the knife and mark the wedges immediately after spreading.