For the lemon ice, boil the sugar with the water, remove from heat, add the lemon zest, and let cool.
Mix the lemon and orange juice into the cooled syrup, and strain.
Freeze in an ice cream maker or in the freezer (stirring and breaking it up with a fork every 30-40 minutes), and at the end stir in the tablespoon of cream.
For the sponge, whisk the egg whites to stiff peaks, gradually add the sugar, then the egg yolks one at a time.
Gently fold in the flour.
Pour into a buttered cake tin and bake at 170°C for about 30-35 minutes.
Once completely cool, hollow out the inside of the cake, leaving about a 2 cm thick wall and base.
Fill the hollowed-out space with the lemon ice, coat the top with whipped cream, and serve immediately so the ice doesn't melt.
Based on: Malatinszky Fanny szakácskönyve (a classic Hungarian cookbook), modernised. Calories and protein are estimated.