Preheat the oven to 400 degrees F (205 degrees C). Have ready two 10-inch cardboard circles. Generously grease a 9-inch springform pan with some of the soft butter, and dust with flour.

To make the cake layers: Beat egg whites in a clean glass bowl with an electric mixer until frothy; gradually add 1 cup sugar. Continue to beat just until soft peaks form.

Beat yolks, milk, lemon zest, vanilla, and salt together in a separate bowl until lighter in color; fold mixture into egg whites. Sift flour over egg mixture, and fold in.

Spread 1 1⁄3 cups cake batter into the prepared pan. Bake in the preheated oven until small, brown spots begin to appear on cake, about 5 to 9 minutes. Remove cake from the oven, and remove layer from pan with a spatula. Dust cake lightly with flour, and place on a rack to cool. Grease pan again, and repeat this process until all of the cake batter is used, about 6 times more. Place the layers between wax paper, and cover with a towel. Chill cake layers for a few hours.

To make the chocolate buttercream: Beat butter and 1/4 cup cocoa powder together in a bowl with an electric mixer on low until light and fluffy. Gradually add milk, remaining 1/4 cup cocoa powder, and vanilla extract; beat until smooth. Add confectioners' sugar, about 2 tablespoons at a time, until desired consistency is reached.

Layer the chilled layers on one of the cardboard rounds with the buttercream. Start with one layer; cover with the buttercream, and then press down with another layer to make a good seal. Repeat this with the remaining layers, but reserve one layer. Wrap the cake in plastic, and chill for at least 6 hours along with the remaining buttercream. Grease the other cardboard round with the shortening, and place the last layer on it.

To make the caramel: Place 1 cup sugar into a non-stick skillet over medium heat. Allow sugar to cook until the edges look melted and brown. Begin stirring with a wooden spoon. Cook until the sugar become an amber color, and is smooth. Carefully pour the caramel over the top of the last layer, and spread to the edges with an oiled knife. Quickly, using an oiled knife, indent the top of the caramel into 16 wedges. Allow to cool slightly, and then retouch the indents with the knife again. Place layer onto a counter top dusted with sugar, and allow the caramel to cool completely.

Place some more buttercream on top of the chilled torte, and top with the caramel round. Frost the sides with the remaining buttercream. Chill the torte before serving.