Preheat oven to 375°F (190°C). Trace six 8-inch circles on parchment paper, flip the parchment over, and place on baking sheets.

Separate the eggs. Beat egg yolks with half of the sugar and vanilla until pale, thick, and ribbon-like.

In a clean bowl, beat egg whites with salt until foamy. Gradually add the remaining sugar and beat to glossy medium-stiff peaks.

Fold the yolk mixture into the whipped whites. Sift in the flour in two additions and fold gently. Fold in the melted, cooled butter without deflating the batter.

Divide batter evenly among the six traced circles and spread each into a thin, even round with an offset spatula.

Bake the sponge rounds in batches for 5-7 minutes each, until lightly golden and springy. Cool briefly, peel away parchment, and transfer to wire racks.

Trim the cooled sponge rounds to neat 8-inch circles if needed. Reserve the flattest layer for the caramel top.

For the buttercream, beat softened butter until smooth and fluffy. Add powdered sugar, cocoa, vanilla, and salt, then beat until fully combined.

Beat in the melted, cooled dark chocolate. Add heavy cream one tablespoon at a time until the buttercream is silky and spreadable.

Place one sponge layer on a serving plate and spread with a thin, even layer of chocolate buttercream. Repeat with four more sponge layers, then frost the sides with a light coat of buttercream.

Place the reserved sponge layer on a parchment-lined heatproof surface. In a small saucepan, cook sugar, water, and lemon juice over medium heat without stirring until the caramel turns deep amber.

Immediately pour the hot caramel over the reserved sponge and spread quickly to the edges. Let it set for about 30-45 seconds, then use a lightly buttered sharp knife to score and cut the caramel-topped layer into 12 wedges before it fully hardens.

Pipe or spoon 12 small buttercream rosettes around the top edge of the assembled cake. Angle one caramel wedge over each rosette to create the classic Dobos Torte finish.

Chill the cake for 60 minutes to set the layers. For the best texture, let it stand at room temperature for 20-30 minutes before slicing and serving.