For Cookies:
Preheat oven to 325°F. Mix butter, brown sugar and vanilla in mixer at medium speed until smooth. Beat in corn syrup and egg. Add 3 cups of flour and salt until blended. If dough is not firm enough to roll into balls, add an additional ½ cup of flour. (Dough can be made ahead of time and refrigerated for later use. Use within 48 hours)

Roll dough into 1-inch balls; roll in walnuts to coat. Place 2 inches apart on ungreased cookie sheet. Press thumb firmly in the center of each cookie to make deep indentation. Follow instructions for chocolate filling. Fill each cookie prior to baking.

For Filling:
Heat corn syrup in 1 quart microwave safe bowl for approximately 1 minute. Add chocolate chips and stir to melt thoroughly. Using a rubber spatula, scrape into pastry bag or 1 quart plastic freezer bag. Seal tightly and snip off a tiny corner at the bottom of the bag. Squeeze filling into thumbprint holes.