Combine the flour and salt in a small bowl, set aside. Cream the butter and sugar on high speed until well-blended. Add the yolks and vanilla and beat once more. Gradually add the flour to the butter mixture and mix until just combined. Stir in chopped nuts, if using. Shape dough into 1-inch balls. Place 2 inches apart on ungreased baking sheets. Carefully use your thumb to make a thumbprint in the middle of each cookie, just enough of a dip to hold the cookie filling. (Do not fill cookies yet.) Bake at 375°F for 8-10 minutes, or until cookie bottoms are very lightly browned. Remove from the oven and cool on wire racks. Once cookies are cool, add your fillings and icing. Your choice of fillings for the thumbprint: candied cherries, melted chocolate, raspberry jam... (NOTE: If using candied cherries, fill cookies halfway through baking.)
Get creative with the filling! Using fresh fruit or fruit preserves makes these cookies look irresistible.
Thanks to JB411 from St. Louis, MO for submitting this recipe.