The yoghurt and sour cream need to be at room temperature. If they're not, heat them a little in a water bath. Beat them with the sugar, add the liquid essence. Dissolve the gelatin in about 3 1/3 tbsp (50 ml) cold water and leave it to swell.
Cut the cake layer down to match the cake form, if it has a hole in it. Wash the strawberries, chop them and arrange them in the form. Dissolve the gelatin in the water bath but do not let it come to a boil. Leave it to cool a bit. Pour it into the yoghurt while stirring nonstop.
If the yoghurt is a bit colder than the gelatin, it's going to start hardening faster but don't worry about that. Pour it carefully over the strawberries, so they don't get moved about and rise up.
Then place the cut prepared cake layer on top. Syrup it lightly with milk or compote. Instead of a cake layer, you can use biscotti or biscuits. Leave it in the fridge for 3-4 hours to gel.
Once it's ready, dip the form in hot water for a few seconds and invert it onto a suitable plate. When serving, pour a topping of strawberries or blueberries on top.