In a large bowl beat butter with electric mixer on medium until softened and creamy (about 30-45 seconds). Add Sugar & Salt. Beat until fluffy.
Add in Egg Yolks & Vanilla. Mix well.
With mixer on low, gradually add in Flour until dough is well mixed. Dough will be very dry and crumbly looking. Cover with plastic wrap and chill in fridge for at least an hour.
After resting dough, preheat oven to 350.
Shape dough into 1 inch balls. Place on ungreased cookie sheet and press down the center of each cookie with your thumb or finger. (Don't press all the way through!)
Bake about 15 minutes, until edges of bottom are starting to brown. Sometimes the holes will fill in a little as they bake. If that happens, I gently press the hole open a little right after they come out of the oven.
Let cool. Fill holes with Jam or Jelly and enjoy! Makes about 5 dozen cookies.