Mix together butter and sugar with a hand beater. Add vanilla and egg yolks and mix to combine. In a separate bowl, mix flour and salt. Add to wet mixture and stir to combine. Cover dough in plastic wrap and cool in fridge for 1 hour.
Preheat oven to 325 degrees.
Beat egg whites in a bowl and fill another bowl with the crushed walnuts.
Form dough into 1 inch balls and dip into egg whites then walnuts (optional).
Place cookies on greased baking sheet and "thumbprint" each ball.
Bake for 15-18 minutes. If using chocolate kisses, place in cookies about 1-2 minutes before taking out of the oven. If using jelly, place in cookies once they are removed from the oven.
Place on cooling rack, let cool and enjoy.
