Preheat your oven to 425°F (220°C). Line baking sheets with parchment paper and mark six 8-inch circles using a pencil.

Whisk the eggs and granulated sugar together in a large mixing bowl until the mixture is light, foamy, and leaves a trail when you lift the whisk. Sift the flour, baking powder, and salt together, then gently fold them into the egg mixture, a little at a time.

Divide the batter evenly between the six marked circles on the parchment paper. Spread the batter gently to fill the circles, ensuring even coverage.

Bake the sponges in the preheated oven for 6–8 minutes, or until they are pale golden and springy to the touch. Remove from the oven, trim the edges with a sharp knife, and let the sponge layers cool on a wire rack.

Whisk the egg whites and confectioners’ sugar in a heatproof bowl set over simmering water until the mixture holds stiff peaks. Cream the butter until soft, then add the egg white-sugar mixture a little at a time while mixing.

Gently melt the dark chocolate in a heatproof bowl over simmering water, making sure the bottom of the bowl doesn't touch the water. Stir occasionally, then let it cool slightly.

Add the melted chocolate to the buttercream mixture and stir until the chocolate is fully blended, creating a smooth, rich frosting.

In a small saucepan, dissolve the sugar in water over low heat. Increase the heat and let it boil until the syrup turns a deep golden color. Remove from heat and let it cool slightly.

Place one of the sponge layers on a serving plate and spread a generous layer of chocolate buttercream over it. Continue stacking the sponge layers with buttercream between each layer.

Pour the cooled caramel over the top of one sponge circle. Once the caramel begins to set, score it into 16 portions using a sharp knife. Place a caramel wedge on each buttercream rosette on top of the cake.