Beat the sour cream and cream cheese with a mixer until you get a cream. Beat the powdered sugar. Beat the yolks and powdered sugar in a water bath and mix until their volume increases 3-4 times.
Add the milk mixture to them and stir with a wooden spatula. Beat the egg whites to snow, add them to the cream and mix carefully again. Add the rum to the coffee.
Soak the biscotti in this mixture. Begin to arrange them in an elongated glass oven dish. Cover them with cream. Arrange the second layer of biscotti perpendicularly to the first.
Finish the tiramisu off with cream. Leave it overnight in the refrigerator and before serving sprinkle it with cocoa.