Start heating oven to 350F. Cream butter until light and fluffy, gradually adding brown sugar. Add egg yolks, mix well. Blend in vanilla, salt, flour. Shape dough into 1-inch balls, dip each in egg white, then nuts. Place on ungreased cookie sheet, about 1" apart. Bake 5 minutes. then with thimble or end or wooden spoon, quickly make depression in each. Bake 8 minutes longer Cook, fill depression with jam.
Makes 3 1/2 dozen