With pastry blender or in a food processor, cut the butter into the flour and baking soda until it resembles coarse meal. Mix in the egg yolk and sour cream to make a soft dough. Wrap in was paper and chill in the refrigerator for at least 2 hours.
Preheat the oven to 350°F. Grease and lightly flour a 10-inch saucepan.
To make the filling, finely grind the walnuts: stir in the sugar and vanilla. Beat the egg whites until stiff but not dry and fold into the nut mixture.
Divide the chilled dough into 3 parts. Roll out one of the parts and transfer it to the pan. Spread with half of the filling, leaving a 1-inch border. Roll out the second part and carefully place it on top of the filling. Cover the remaining filling. Roll out the last part and place it on top. Tuck the edges under slightly to seal. Flatten the pastry to conform the sides of the pan. With a sharp knife, score the top of the pie lightly into diamonds.
Brush the tops of the pieces with beaten egg yolk and bake for 45 to 50 minutes, until browned. Cut into diamonds to serve.
Note: Unlike its close cousin baklava, it is made with baking soda and sour cream dough rather than phyllo.