Combine and sift the dry ingredients: flour, cocoa and baking soda.
Beat the eggs with sugar using a mixer, add the sour cream and melted butter and beat again.
Gradually stir in the dry ingredients with a spoon, then work the dough by hand until it reaches a modeling-clay consistency, adding more flour if needed.
Divide the dough into pieces (I get 6 balls). Roll out each one very thin on parchment paper, trim to shape and prick with a fork.
Bake the layers at 180°C for 5-7 minutes each - they bake very fast. This makes about 7 layers.
For the filling, beat the room-temperature butter until pale, add the boiled condensed milk and beat again, then add the cocoa and beat once more.
Spread the filling over all the layers, including the top and sides, and generously grate chocolate on top.
Leave the cake on the counter for a couple of hours, then refrigerate overnight. Cut into pieces in the morning.