Assemble the cake layers:
Level the cakes, if necessary, and slice each cake into two layers making a total of four layers of cake.
Place the bottom layer on a flat serving platter or a cake stand lined with strips of waxed paper to keep it clean while assembling the cake. Top the layer with a scant 1-1/2 cups buttercream, spreading it evenly with a metal cake spatula almost to the cake’s edge. In a small bowl, stir together the marmalade and Grand Marnier. Spread a third of the marmalade mixture on the next cake layer, then lay it, marmalade-side down, over the buttercream filling. Repeat with the next two layers.
Frost the cake:
First apply a light coat of frosting (called crumb coating) to seal the cake crumbs in: Spoon about 1/2 cup buttercream into a small bowl. Spread it in a very thin layer over the entire cake with a small metal cake spatula. You should be able to see the cake layers through the icing. Chill the cake for about 20 minutes or until the icing is firm.
Spread the remaining buttercream thickly and evenly over the entire cake with a large metal cake spatula. Don’t worry about getting a smooth, perfect finish; just make sure the cake is completely covered and the frosting is spread uniformly. You shouldn’t be able to see the layers underneath the buttercream.
With the back of a teaspoon, smear the icing and pull it upward to form curls and swirls over the entire cake. Pile the chocolate curls on the top of the cake to garnish.