Prepare sponge batter: Beat egg yolks with sugar until pale and fluffy. Add vanilla. In another bowl, beat egg whites with a pinch of salt until stiff peaks form. Gently fold egg whites into yolk mixture, then fold in flour.
Bake thin layers: Divide batter into 6–7 portions. Spread each thinly on parchment paper in round shapes (about 20–22 cm). Bake each layer at 180°C (350°F) for 6–8 minutes until lightly golden. Cool completely.
Make buttercream: Beat butter with powdered sugar until fluffy. Add melted chocolate and vanilla. Mix until smooth.
Assemble cake: Stack layers, spreading chocolate buttercream between each one. Leave the top layer plain for caramel.
Prepare caramel: Melt sugar with lemon juice in a pan until golden amber. Work quickly.
Create caramel top: Pour caramel over the top layer and spread evenly. Before it hardens, cut into wedges with a greased knife.
Finish cake: Place caramel wedges on top of the cake. Spread remaining buttercream around the sides.
