Resift flour with baking powder, baking soda and salt.
Cut in finely, butter or margarine.
Combine egg yolk with the sour cream and add to flour mixture.
You may not need the entire amount—if not, brush the tops of the scones with the remainder.
Roll to about 3/4 of an inch thickness, and cut out with a 3 inch cookie cutter or, more traditionally, into one large round, cut into quarters.
Bake at 400° for 12 to 15 minutes.