Prepare the Sponge Mixture: Sift the all-purpose flour and potato starch or cornstarch together, then set aside. Reserve two-thirds of the granulated sugar. Whisk the egg yolks with the remaining sugar until the mixture becomes light and pale and reaches ribbon stage, meaning the batter leaves a visible trail when the beater is lifted. Stir in the lemon zest. Whip the Egg Whites: In a separate bowl, whisk the egg whites to soft peaks. Gradually add the reserved sugar while whisking until the whites become stiff and glossy.
Combine the Batter: Stir two large spoonfuls of the whipped egg whites into the yolk mixture to loosen it. Gently fold in the sifted flour mixture. The batter will initially seem thick and unusual, so continue mixing carefully until no streaks of flour remain. Fold in the remaining egg whites as gently as possible to preserve the incorporated air. Prepare the Pans: Preheat the oven to 425°F. Spray two 8-inch round cake pans with flour-based baking spray, or grease them and line the bottoms with parchment rounds.
Divide the Batter: Using a kitchen scale, divide the batter evenly among six portions. The source portioned approximately 3.30 oz. per layer, although the exact weight may vary. Alternatively, divide one-third of the batter between the two prepared pans. Spread each portion into an even layer across the bottom using a small offset spatula or the back of a spoon. Bake the Sponge Layers: Bake for 5–6 minutes, or until the sponge is golden. Immediately invert the cakes onto wire racks and remove the parchment if used.
Repeat the Baking: Wash and re-grease the pans, then repeat the process twice more with the remaining batter. Continue until you have six thin sponge layers in total.
Make the Simple Syrup: Combine the granulated sugar and water in a small saucepan. Bring the mixture to a boil and cook until the sugar has completely melted. Remove from the heat, stir in the vanilla extract, and allow the syrup to cool completely.
Melt the Chocolate: Place the chopped semisweet chocolate in a microwave-safe bowl. Heat it in 30-second increments, stirring as needed, for about 2 minutes total, until melted and smooth. Allow the chocolate to cool to room temperature while keeping it liquid and pourable. Whip the Butter: Place the room-temperature butter in the bowl of an electric mixer fitted with the whisk attachment. Beat on medium-high speed for about 2 minutes, until light and fluffy.
Build the Buttercream: Add the egg yolks one at a time, mixing well after each addition. Add the cooled melted chocolate and confectioners’ sugar. Scrape down the sides of the bowl to incorporate any butter or chocolate that may have collected there. Add the salt and vanilla extract, then beat until the buttercream is fluffy. Prepare the Piping Bag: Transfer about 1 1/4 cups of the chocolate buttercream into a piping bag fitted with a large closed star piping tip. Set aside for decorating the finished cake.
Assemble the Layers: Lightly brush one sponge layer with the cooled simple syrup. Spread a 1/4-inch-thick layer of chocolate buttercream over it. Place another sponge layer on top and repeat the syrup-brushing and buttercream filling process until all six sponge layers are stacked. Frost the Cake: Spread the remaining chocolate buttercream evenly over the top and sides of the assembled cake.
Add the Almonds: While the buttercream is still tacky, gently press the sliced almonds onto the sides of the cake so they adhere to the frosting.
Pipe the Rosettes: Using the reserved buttercream, pipe 8 large rosettes evenly around the top edge of the cake.
Chill the Cake: Refrigerate the assembled cake for 1 hour before adding the caramel garnish.
Mark the Caramel Circle: Draw an 8-inch circle on a sheet of parchment using the same cake pan used for the sponge layers as a guide. Place the parchment on a flat, heat-proof work surface.
Prepare the Caramel: Combine the granulated sugar and lemon juice in a small saucepan, mixing until the sugar resembles wet sand. Cook over medium-high heat, watching constantly and occasionally stirring or prodding the mixture, until it develops an amber caramel color. Spread the Caramel: Immediately pour the hot caramel into the center of the marked circle. Quickly spread it into the circle using a hot stainless-steel spatula. Heat the spatula by resting it on a hot stove eye or warming it with a chef’s torch.
Score the Caramel: Allow the caramel to sit for a few seconds, just until it begins to set. Grease a large chef’s knife, then carefully score the caramel into 8 wedges.
Break the Caramel Wedges: Let the caramel set completely, then gently break it apart along the scored lines to create 8 wedges.
Decorate the Cake: Rest one caramel wedge at an angle on each of the 8 buttercream rosettes, creating the characteristic fan-style caramel decoration.
Store the Torte: Keep the finished cake covered at room temperature. The source notes that the cake keeps well for up to a week and can improve in flavor and texture as it stands.
