First, make the layers. Mix the sugar and yoghurt in a bowl and beat. Add the rest of the products, excluding the cocoa, and beat well. If you have 3 identical round trays of the same diameter, distribute the mixture in them evenly. Leave 2 of the white mixtures as is but to the 3rd add the cocoa, stir well and pour into the final tray. The trays need to be covered with oiled baking paper if they don't have a nonstick coating.
If you don't have 3 identical trays, bake the layers subsequently 1 by 1 using the same tray. Be sure to use new baking paper for each one. Bake at 356°F (180 °C) until ready (check with a toothpick by poking them). After baking, leave the layers to cool.
Making the cream:
Set aside 1/2 cup of milk. Pour the rest in a pot on the stove and put it to heat up. In the set aside milk, dissolve the starch and sugar. Beat the yolks separately with the butter. Once the milk reaches boiling point, pour the dissolved starch in it and stir energetically to prevent burning.
Once the cream thickens, remove it from the stove and add the beaten yolks and butter to it. Stir well and leave the prepared cream aside, covered with foil, to prevent a crust from forming.
Make the syrup by pouring the sugar, water and lemon juice in a small pot on the stove and boiling until thickened.
Assembling the cake:
Place 1 of the light layers in a cake platter or large, flat plate. Syrup it evenly, smear generously with cream, place the cocoa layer, syrup, smear with cream, place the other light layer and syrup. Since it's the last one, smear it with cream evenly and make sure it looks aesthetically pleasing.
For the decoration, beat the liquid and cover the cake with it (over the other cream).
Sprinkle with colorful sugar sprinkles.
Put the cake in the fridge for at least 4-5 hours before serving.