Preheat oven to 350°F. Cream together butter and sugar. Beat egg yolk well and add lemon juice. Add alternately to creamed mixture with flour. Combine until well-blended. Shape dough into balls about 1 inch in diameter. Beat egg white slightly and dip each ball into egg white. Roll in chopped walnuts.
Place on greased cookie sheet. Make fairly deep indentation in centre of each cookie. Bake for 5 minutes and then quickly indent the centre a second time. Bake for another 10 to 12 minutes. Remove from oven and fill with Pure Labrador Spread of your choice.