Preheat oven to 350 degrees F. Combine flour, salt, and baking soda in a separate bowl, set aside. Cream together the butter and 2 cups brown sugar. Add the eggs and vanilla. Mix well. Add the flour mixture a little at a time, mix well. The mixture will be stiff. In a separate bowl mix together the walnuts, 1 cup brown sugar and the sour cream (this is your filling).
Shape the dough into 1 inch balls, and place 2 inches apart on an ungreased cookie sheet. Make a thumb print depression in the cookie dough. Place 1 tsp. of filling into each depression.
Bake for 12-14 minutes or until edges are golden brown. Let cool on cookie sheet for 1 minute. Remove to cooling rack to finish cooling.