This is a Gaston Lenôtre classic, but I’ll be using components from the blog here to put it together. The original recipe uses one of those old foil cake layer pans — the kind you can sometimes still find in supermarkets — as a mold. You can use a 9″ round cake layer pan as a substitute, if you have one with sloping sides so much the better. In an ideal world the oranges should be sliced on a mandoline, since that gives the best presentation. If you don’t have one, a steady hand works almost as well. You’ll need:
First, prepare the oranges. Slice them very thin, about 1/16″. Combine the sugar and water in a medium saucepan and bring the mixture to a boil. Add the orange slices and simmer for 2 hours. Pour the mixture into a bowl and let it sit on the counter overnight.
Combine about 2/3 cup of the orange syrup you’ve made with the Grand Marnier. Set six or seven of your best orange slices aside for decorating and roughly chop what’s left. Combine the pastry cream and whipped cream. Separate the mixture in two, and fold the chopped orange pieces into one half.
Line the cake pan with plastic wrap, then with orange slices, arranged in a decorative pattern. Fill the mold half way with the diplomat cream (without orange slices). Cut the génoise cake layer in two and brush each half with the Grand Marnier syrup. Place one of the layers into the mold. Cover the layer with the diplomat cream that has the chopped orange in it. Lay on the other piece of génoise, fill in the edges with more of the non-chopped-orange diplomat cream. Put a cake circle or plate on top and press down gently but firmly to compact it. Leave the plate on the cake and refrigerate it for at least two hours.
When you’re ready to unmold the cake, place a serving platter on top and flip everything over. Remove the mold, then very gently the pastil wrap. Brush the orange slices with remaining syrup to make them shiny. The cake will hold for up to 48 hours.