In a large mixing bowl whip the mascarpone, powdered sugar and about half of the sour cream with an electric mixer.
In another bowl whisk the rest of the sour cream together with the Nutella.
Pour the milk in a bowl bigger than the size of the crackers/ biscuits. Dip each cracker for just few seconds (harder cookies like the Bulgarian plain biscuits might need to be dipped for 15-20 seconds). Arrange a layer of crackers/ biscuits on the bottom of a 11 x 7 inch/ 28 x 18 cm pan or similar. You may need to break some of them to fit.
Spread on top about ⅓ of the mascarpone filling. Smooth it out with a spatula.
Spread ⅓ of the chocolate mixture. It doesn't need to cover well the filling.
Continue the process two more times.
Cover the top of the pan with plastic wrap. Refrigerate 6-8 hours or overnight.
Serve sprinkled with cracker crumps. You can decorate the cake with berries but even plain it is just fine.
Kinds of cookies or biscuits that you can use for this dessert: graham crackers, petit beurre, shortbread cookies or any plain rectangular cookies/ biscuits. Alternatively you can use vanilla wafers and arrange them in a round cake pan. Digestive biscuits can work too but you need to break them into small pieces after dipping them in the milk.
The pan size is indicative. You can potentially use a smaller pan and make 4 layers of crackers/ biscuits + filling, if it is tall enough. Test first by layering only the crackers, so you know in how many parts to divide the fillings.
You may need to warm up the Nutella, before mixing it with the sour cream.
This cake is best served 15 minutes after being removed from the fridge.