For the crust: Cut the butter into the flour, sugar and orange zest until the size of peas. Mix in the egg yolks and form a smooth ball without kneading too much. Wrap in plastic wrap and chill for one hour.
For the filling: In a large saucepan, bring the milk, sugar and vanilla to a boil. Remove from heat and add farina, stirring constantly, then poppy seeds. Place back on heat and boil again, briefly. Remove from heat and let the mixture cool down.
While mixture is still warm, stir in the butter until melted.
(Do not beat the egg whites until you are ready to fill the crust.)
For the topping: Beat all egg whites until stiff. Mix all the sour cream topping ingredients together, except the egg whites, until smooth. Fold in half the egg whites. Fold the other 2 egg whites into the cooled poppy seed mixture.
To assemble the cake: Roll one half of the dough into an 11-inch circle and cut to fit bottom of 11-inch springform pan.
Roll out 1/4 of the dough into a rope and flatten into a 1 or 1-1/2-inch rim. Press around the edge of the springform and attach to the base with a little water or egg white. Continue with rest of dough. Trim crust evenly for a nice presentation.
Fill crust with poppy seed filling and then top with sour cream topping, making sure to cover to the edge of the crust seal.
Bake in a preheated 350-degree oven for 50-60 minutes or until cake is browned and firmly set.