In a large bowl mix the flour and white sugar together. Cut in the butter with a pastry blender or two knives until like breadcrumbs. Stir in the unbeaten egg and mix with your hands until dough holds together.
Divide dough into 7 equal parts. Roll each part into a 23cm circle on a lightly floured non-stick baking tray. (The baking trays need to be without edges so you can roll out the dough on the baking tray as you can't transfer them after they're rolled out.) If the edges get uneven while rolling out, just push them back and roll again until you get a fairly even circle.
Bake for 10 to 12 minutes, until lightly browned on edges. Cool and remove gently.
Filling: Combine sour cream, icing sugar, chopped walnuts and vanilla then mix until smooth. Divide filling into 6 equal portions.
To Assemble: Stack the cake layers while putting the filling between each cake as you go. The 7th layer sets on top of the stack; there is no sour cream filling on top of it. Sift icing sugar on the top layer.
Refrigerate for at least 5 hours or longer so the whole thing can meld and mellow together into a "cake-like" result.