Drain the canned berries or cherries and save the juice. Measure 250 ml/ 1 cup of the juice. Add water if you don't have enough juice. Place the liquid in a small pot and bring to a boil.
Meanwhile, stir the pudding powder or corn starch mixture in a small bowl with 2 or 3 tablespoons cold water. You should have a thick yet still runny paste.
When the juice starts to boil, take it off the heat and whisk in the pudding or corn starch mixture. Place back on the heat and bring to a boil again while whisking all the time. Let bubble once or twice. Remove from the heat, stir in the berries or cherries (and 1 teaspoon vanilla extract if using corn starch instead of pudding powder), and leave to cool down a little.
Place one cake on a cake platter and put a cake ring around it (or a rectangular adjustable frame for the cake baked on the baking tray). Pour the berries on top and level. Let them get completely cool before adding the rest.
When the berries are completely cool, soak the gelatin leaves in cold water for about 10 minutes or according to the packet's instructions.
In the meantime, mix together the cream cheese, lemon juice, and zest and the 50 g sugar in a bowl.
Squeeze the gelatin gently, place it in a pan (large enough to hold the cream cheese mixture as well), and dissolve it on very low heat, stirring continuously. Do not let it boil or get too hot, it will only take seconds until the gelatin is dissolved, so don't leave it unattended.
Remove from the heat, add one tablespoon from the cream cheese mixture and stir until fully incorporated. Add another tablespoon of the mixture and incorporate it completely again. Repeat until all the cream cheese mixture is incorporated.
Place the pot with the filling in the fridge. Check often and when the mixture STARTS to thicken, whip the heavy cream until stiff and fold it carefully into the cream cheese mixture. Pour this mixture on top of the berries and level nicely.
Cover with the second cake, press it down gently, and place the cake in the fridge for several hours, ideally overnight. Remove the metal ring before serving and slice the cake. You can sprinkle it with some icing sugar if you like.