Preheat oven to 375 degrees.
In a large bowl, beat butter, sugar, egg yolk and vanilla until creamy. At low speed gradually add flour and salt and mix until moist.
Spread walnuts on sheet of waxed paper.
With lightly floured hands, roll dough into small balls (about 1 tablespoon).
Mix egg white in a small bowl. Dip cookie balls, one at a time, into the egg white and then roll in walnuts.
Place on an ungreased cookie sheet.
Bake for 5 minutes.
Meanwhile, in a small bowl mix jam with a fork until spreading consistency.
Working quickly, remove cookies from oven and, with the end of a wooden spoon, make a well in each cookie. Fill with 1/2 tsp. jam.
Return cookies to oven and bake 10 minutes.
Cool on wire rack.