Prepare cake layers: Beat egg yolks with half the sugar until pale. Beat egg whites with remaining sugar to stiff peaks. Fold together, then gently fold in flour and vanilla. Divide into 6 thin layers and bake at 180°C (350°F) for 8–10 minutes each. Cool completely.
Make buttercream: Beat butter and sugar until fluffy. Add melted chocolate and vanilla, mix until smooth.
Assemble cake: Spread chocolate buttercream between each layer, stacking evenly.
Caramel top: Melt sugar with water until golden caramel. Quickly pour over the top layer and spread evenly. Let set slightly, then cut into wedges before fully hard.
Finish: Place caramel wedges on top of the cake. Chill before serving.
