Mix together thoroughly: sugar, butter, egg yolks, and Vanilla Extract.
Sift together and flout and salt and stir into butter mixture until just combined.
Roll into one inch balls.
Dip into slightly beaten egg whites, foamed a little with a fork.
Dip the dough balls into egg whites, and then roll into finely chopped walnuts.
Place about 1 inch apart on an ungreased baking sheet.
Bake 5 minutes at 375 degrees.
Remove from oven and quickly press thumb into each cookie to make a small well.
Return to oven and bake 8 more minutes.
In the thumbprints put a little bit of raspberry jelly or jam.