Preheat oven to 350 degrees F. Beat the egg yolks and sugar until pale and creamy. Mix in the orange zest. Sift together both flours to aerate well and add to the yolk mixture. Beat the egg whites in a separate bowl until they are firm and well peaked. Fold the whipped egg whites into the yolk mixture lightly and quickly. This quantity yields 6 layers.
Spread a thin coating of mixture in the bottom of a greased and floured 9 1/2-inch spring-form pan and smooth carefully. Bake immediately for about 5 to 8 minutes. Bake 2 layers at a time if you have the pans. When the cakes are lightly browned, turn into a rack and continue with the other layers.
As soon as the filling is finished, set aside the best looking cake layer for the top, and sandwich the rest together with the chocolate buttercream, spreading it over the top and sides. Brush any loose crumbs off the reserved top layer and lay it on a large sheet of waxed paper.
Chocolate Buttercream: With an electric mixer, beat the shortening and butter together until blended. Turn the mixer down to the lowest setting and gradually add the confectioners' sugar and the sifted cocoa powder. Turn the mixer back up and beat at high speed until light and fluffy, about 2 minutes. Gradually beat in the vanilla and corn syrup until incorporated. Add the cream and refrigerate.
Caramel Glaze: Add water, sugar and butter to a saucepan over a high flame. Bring to a boil and cook until golden amber.
Quickly pour the caramel straight over the reserved top layer and smooth it out using a clean and dry long offset spatula. Using a lightly buttered long knife, immediately score the top of the caramel layer into 10 sections, cutting through the caramel glaze, but not through the cake. Leave to cool. Lay the caramel layer on top of the filled cake layers. Do not refrigerate. Store at room temperature.