For the blueberry sauce, mix together 2/3 cup (150 g) sugar, 1/2 tsp corn starch, the cinnamon, 1/8 teaspoon salt and 2/3 cup (160 ml) water in a pot. Once it comes to a boil, add 10.5 oz (300 g) of blueberries and stir until thickened. Finally, add the vanilla, lemon juice and grated lemon rind.
Leave the blueberry sauce to cool slightly and add the gelatin, which you've dissolved in water and liquefied ahead of time. Leave it as is, while occasionally stirring and in the meantime prepare the cake layer and cream.
For the layer, beat the egg whites to snow and gradually add the sugar. Whisk for a few minutes and add the egg yolks and the hot water. Sift together the flour and starch and carefully add them to the mixture. Also pour in the melted and cooled butter slowly and stir until well mixed. Do not worry if the mixture deflates a bit.
Bake this layer a lightly oiled and floured baking dish in a preheated 356°F (180 °C) oven. To prepare the cream mixture, first soak the gelatin in 2/5 cup (100 ml) of water to swell, then liquefy it slightly on the stove for about 30 seconds. Beat the liquid cream to fluffiness and then stir in with the sour cream which you've sweetened with the powdered sugar.
Add the vanilla, cooled gelatin and stir slightly. Distribute the cream onto the baked cake layer, which you can poke some holes in with a fork. Top with the cooled, but not yet gelled, blueberry sauce. Leave the cake in the refrigerator until completely tightened.