Make the Sponge Cake Layers:

1. Prepare Pans: Preheat oven to 375°F (190°C). Line the bottoms of 2-3 nine-inch round cake pans with parchment paper circles. Do not grease the sides (the batter needs to cling as it rises). If you only have 2 pans, you’ll bake in batches.
2. Whip Egg Whites: In a large clean bowl, beat egg whites with an electric mixer on medium speed until foamy. Gradually add ½ cup of sugar, one tablespoon at a time, beating on high speed until stiff, glossy peaks form (about 5-7 minutes). The meringue should be thick and hold its shape.
3. Beat Egg Yolks: In a separate bowl, beat egg yolks with remaining ¼ cup sugar and vanilla extract until thick, pale yellow, and tripled in volume (about 3-4 minutes).
4. Fold Together: Gently fold the egg yolk mixture into the whipped egg whites using a rubber spatula. Fold in large, sweeping motions to maintain airiness.
5. Add Flour: Sift the flour and salt over the egg mixture in three additions, gently folding after each addition just until no flour streaks remain. Don’t overmix or you’ll deflate the batter!
6. Portion Batter: Divide batter evenly among prepared pans (about 1 cup per pan for thin, even layers). Spread gently with an offset spatula to create even thickness.
7. Bake: Bake for 8-10 minutes, or until the tops spring back when lightly touched and edges just begin to pull away from the pan. Don’t overbake or layers will be dry!
8. Cool: Run a knife around the edges immediately and invert onto a wire rack. Peel off parchment and let cool completely. Repeat with remaining batter if baking in batches. You need 6 layers total.

Make the Chocolate Buttercream:

9. Melt Chocolate: Melt dark chocolate in a double boiler or microwave in 30-second intervals, stirring between each. Let cool to room temperature but still liquid.
10. Beat Butter: In a large bowl, beat softened butter on medium-high speed for 3-4 minutes until light, fluffy, and pale in color.
11. Add Egg Yolks: Beat in egg yolks one at a time, mixing well after each addition. The mixture may look slightly curdled—this is normal!
12. Add Sugar: Gradually beat in sifted powdered sugar on low speed, then increase to medium-high and beat for 2 minutes until fluffy.
13. Add Chocolate: With mixer on low, slowly pour in the cooled melted chocolate. Add vanilla, salt, and milk/cream. Beat on medium-high speed for 2-3 minutes until smooth, silky, and spreadable. If buttercream is too thick, add more cream one teaspoon at a time.

Make the Caramel Top:

14. Prepare Surface: Lightly oil a baking sheet or marble slab and have an oiled knife ready.
15. Cook Caramel: In a heavy-bottomed saucepan, combine sugar and lemon juice. Cook over medium heat without stirring (you can swirl the pan gently) until sugar melts and turns deep golden amber (about 8-10 minutes). Watch carefully—it can burn quickly!
16. Add Butter: Remove from heat and carefully stir in butter. The mixture will bubble vigorously—be careful!
17. Pour and Score: Immediately pour caramel onto the oiled surface and spread quickly to a thin circle slightly larger than your cake layers. Let cool for about 30 seconds, then use an oiled knife to score the caramel into 12-14 triangular wedges (like cutting a pizza). Work quickly—caramel hardens fast!
18. Cool Completely: Let caramel cool and harden completely (about 15 minutes), then carefully break along scored lines into wedges.

Assemble the Cake:

19. Trim Layers: Using a serrated knife, trim any uneven edges from cake layers so they’re uniform.
20. First Layer: Place one cake layer on a serving plate or cake board. Spread with a generous layer of chocolate buttercream (about ⅓ cup).
21. Stack Layers: Continue layering: cake, buttercream, cake, buttercream, repeating until you have 5 layers stacked. The 6th cake layer will be the top.
22. Reserve Buttercream: Before placing the top layer, reserve about ⅓ cup buttercream for decorating the sides if desired.
23. Top with Final Layer: Place the final cake layer on top (smooth side up).
24. Frost Sides: Use remaining buttercream to frost the sides of the cake in a thin layer. You can leave the top unfrosted or spread a very thin layer.
25. Add Caramel Shards: Arrange the caramel triangles on top of the cake in a circular pattern, standing them up slightly for dramatic effect or laying flat.
26. Chill: Refrigerate for at least 2 hours before serving to allow buttercream to set.