Peel and thinly slice the oranges. Arrange them in a bowl, sprinkle with sugar, and let them sit for at least 1 hour.
Mix the yolks and sugar with an electric mixer, until light and fluffy. Add the flour with baking powder to the egg yolk mixture. Gently whisk until the flour are fully incorporated and you have a smooth batter.
Beat the large egg whites until they form stiff peaks. Gently fold this foam into the egg yolk cream, taking care not to deflate it.
Pour the batter slowly and carefully into the greased spring form tin. Use a gentle touch to level it evenly, avoiding any rough movements or shaking.
Bake at 180°C (350°F) for 22-25 minutes. The cake is done when it has a golden brown colour and a toothpick inserted into the middle comes out clean.
In a saucepan, heat the milk with vanilla extract and half of sugar.
In a bowl, whisk together the egg yolks with the rest of sugar until light and creamy.
Whisk in gradually 100 ml (½ cup) of the warm milk into the egg mixture, then add in the corn starch until fully incorporated.
Pour the mixture back into the saucepan over the warm milk and cook over medium heat, stirring constantly, until thickened and bubbly. Continue cooking for 1-2 minutes more to ensure the starch is cooked through.
Add in one orange zest and let the pastry cream cool to room temperature.
Whip the heavy cream to stiff peaks. Gently fold the cooled pastry cream into the whipped cream until just combined, being careful not to overmix.
Choose a round pot or dish, ideally a little wider than your sponge cake and line it with kitchen foil.
Arrange the orange slices on the bottom and bottom edge of the pot. Spread a third of the diplomat cream over the orange slices, to ensure it fills the gaps and reaches the spaces between them.
Place the sponge cake layer you previously cut into three thin pieces on top of the cream. Drizzle few spoonful of strained orange juice over the cake.
Repeat layering with diplomat cream, orange pieces, and sponge cake slices until all ingredients are used.
Wrap the dish tightly in kitchen foil and refrigerate for 24 hours to allow the flavours to meld and the cake to set.
Carefully unwrap the foil and invert the cake onto a serving plate. Remove the pot and any plastic film, revealing your beautiful orange cake in all its glory.
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Notes
You can also mix this batter in a different order: first, whisk egg whites with sugar until stiff peaks form. Then, gradually whisk in one egg yolk at a time, until fully incorporated. Finally, gently fold in flour and baking powder.When you bake the sponge cake, do not open the oven door for the first 10 minutes.In the pastry cream, instead of starch, you can use gelatine as in the original recipes.