Combine 1 cup water, 1 cup sugar, and 1 tablespoon orange zest in a saucepan. Bring to a boil, and stir until the sugar dissolves. Cool completely.
Beat 5 egg whites until stiff peaks form.
In a separate bowl, beat 5 yolks until thick and pale, gradually adding 1 tablespoon water, a pinch of salt, 1 tablespoon sugar, and 1 tablespoon oil.
Fold in 5 tablespoons of potato starch, one tablespoon at a time. Add 1 tablespoon of orange zest.
Fold in the egg whites.
Spread the batter onto a parchment-lined jelly-roll pan.
Bake for 10-12 minutes, until set. Cool.
Cut the cake into four even strips.
Melt the chocolate with 1 tablespoon of oil. Whisk in 5 yolks.
In a separate bowl, beat 5 whites until stiff, gradually adding ½ cup sugar.
Fold the egg whites into the chocolate mixture.
Cover a 9 x 13 or English cake pan with plastic wrap, letting the overhang drape over the sides. Place parchment at the bottom.
Layer one cake strip in the pan, brush it with syrup, spread a mousse layer, repeat twice. Top with the final cake strip, and brush it with syrup. Freeze for 4 hours.
Heat ½ cup of cream until steaming. Pour it over the chopped chocolate, and let it sit for 1 minute. Whisk until smooth. Cool 5 minutes.
Spread the ganache over the top layer of the cake. Garnish with ¼ cup candied orange cubes.