Clean the apples, peel their skins, and remove the cores. Cut the apples into cubes about ½-1 inch in size.
Put the sliced apples in a pot and cover them with a cup of water. Increase the heat, cover with a lid, and bring to a boil. Then turn the heat down to low and let the apples simmer under the lid for 15 minutes.
Add ground cinnamon, sugar, and lemon juice to the cooked apples. Stir to combine.
Pour a cup of water into a bowl and add a ladleful of the cooking juices from the apples. Stir in the pudding powder or cornstarch and whisk well, so there are no lumps in the mixture.
Slowly pour the pudding powder mixture over the cooked apples. Stir thoroughly and bring to a boil over medium heat. The custard powder will thicken the apples. Cook over medium heat for about a minute, and then remove the pan of apple jelly from the heat source.
Crush the apple pieces a bit, for example, with a potato masher. Do not blend the mixture; you should still see small pieces of apple chunks in it.
Take a round springform cake pan with a diameter of 9-10 inches. Line the bottom with baking paper, fit the sides and simply cut the excess paper around the perimeter with scissors. You can grease the bottom with baking paper with a little unsalted butter.
Place a layer of biscuits on the bottom of the springform pan, one right next to the other.
Scoop the apple jelly with a ladle and spread it over the biscuits while smoothing the surface.
Place another layer of biscuits on top of the jelly.
In a medium bowl, mix the sour cream with the sugar. Pour the cream mixture over the biscuits and spread evenly. Scrape the sides and bottom of the bowl with a rubber spatula to make the most of the cream.
Now put the cake in the fridge and leave it there overnight. The cake flavors will develop, and the biscuits will soften as they soak up the juices from the apple jelly.