Difficulty: 3/5 (Intermediate)
As a professional baker with over a decade crafting citrus desserts, this orange cake delivers a perfect balance of moist sponge, tangy filling, and glossy icing every time.
Prepare the cake base: Grate the zest from the 2 untreated oranges. Combine it in a saucepan with 20 g sugar and 20 cl water. Bring to a boil and simmer for 15 minutes, then drain.
Beat the 6 eggs with the remaining 180 g sugar until pale, smooth, and ribbon stage (yolks and whites can be combined here for simplicity, or separate for extra lightness). Gently fold in the drained zest, sifted flour, and potato starch.
Preheat oven to 180°C (thermostat 6).
Whip the egg whites to stiff peaks in a separate bowl, then delicately fold into the batter.
Butter a cake tin and pour in the batter, filling no more than 2/3 full. Bake for 30-40 minutes. Test doneness with a knife blade—it should come out clean.
Meanwhile, make the filling: Cut 100 g butter into small pieces and juice the 2 untreated oranges. Whisk the 2 eggs in a bowl, then add butter pieces and orange juice.
Place over a double boiler (bain-marie) and stir gently until thickened and smooth, about 30 minutes. Cool completely.
Remove cake from oven, unmold onto a wire rack, and cool fully. Slice horizontally in half. Spread the lower half generously with orange filling, then replace the top.
For the frosting: In a bowl, gradually mix 3 egg whites with 500 g icing sugar using a spatula until creamy. Place over bain-marie, add a few drops of lemon juice, and stir until it thickens. Remove from heat immediately.
Coat the cake evenly with spatula. Let set.
Decorate with thin slices from 1 untreated orange and 4 strips of long candied zest.
