The day before, prepare the oranges:heat the sugar (300 g) and 30 cl of water in a large saucepan. Wash the oranges by brushing them under hot water, cut them into slices, put them in the pan, leave to simmer for 30 minutes then remove from the heat and leave to cool in the syrup.
Prepare the cream:Bring the milk to a boil in a heavy-bottomed saucepan. Whisk the egg yolks and the remaining sugar until the mixture whitens and doubles in volume. Add the cornstarch, pour in the hot milk in a thin stream while stirring, transfer everything to the saucepan and thicken over low heat for about 5 minutes.
Soak the gelatine sheets in cold water to soften them, drain them and throw them out of the heat into the hot cream. Stir, they melt immediately. Let cool. Whip the fresh cream into Chantilly, and mix it with the cooled cream at the same time as the Grand-Marnier.
Drain the oranges, reserving the cooking syrup. Melt the marmalade over low heat with 4 tablespoons of orange cooking syrup. Line a round bowl with cling film and cover with slices of candied orange.
Cut the sponge cake into two discs and put half the cream on the oranges. Lay a disc of sponge cake, soak it in lukewarm marmalade and fill with the rest of the cream. Finish with the other disc after brushing it with marmalade.
Cover with cling film and chill for 12 hours.
To serve, unmold the dome on a dish and drizzle with a little orange cooking syrup.
