As a seasoned pastry chef, I love navel oranges for their sweet, juicy flavor and complete lack of seeds—perfect for elegant desserts like this stunning dome cake.
The day before, candied the oranges: In a large saucepan, heat 300 g sugar with 30 cl water. Scrub the oranges under hot water, slice them, add to the pan, and simmer for 30 minutes. Remove from heat and cool in the syrup.
Prepare the pastry cream: Bring milk to a boil in a heavy-bottomed saucepan. Whisk egg yolks with remaining 150 g sugar until pale and doubled in volume. Stir in cornstarch, then gradually add hot milk while whisking. Return to the saucepan and cook over low heat, stirring, until thickened, about 5 minutes.
Soften gelatin sheets in cold water, drain, and stir into the hot pastry cream off the heat until melted. Cool slightly. Whip cold heavy cream to chantilly consistency, then fold into cooled pastry cream along with Grand Marnier.
Drain oranges, reserving syrup. Warm marmalade with 4 tablespoons reserved syrup. Line a round bowl with plastic wrap, arrange candied orange slices inside.
Cut sponge cake into two discs. Spread half the cream over oranges, top with one disc soaked in warm marmalade syrup. Add remaining cream, then top with second disc brushed with marmalade.
Cover with plastic wrap and refrigerate 12 hours. Unmold onto a serving dish and glaze with reserved orange syrup for a glossy finish.
