1) Cream the butter and sugar on high speed for about 3 minutes.
2) Separate the eggs. Add the yolks and vanilla extract to the butter mixture. If using nuts place the egg whites in a shallow dish on the side and whisk them until bubbly and frothy (the egg whites will be used to keep the nuts on the cookies).
3) Add the flour and salt. Mix until just combined. Place the dough in the fridge for 30 minutes and preheat the oven to 350F.
4) Roll the dough into balls about 1 inch in diameter. If using nuts, dip the balls into the egg whites then roll them into the nuts until covered. Place the balls on parchment lined cookie sheets.
5) Press down with your thumb to make a small well in the center of the cookie. Do not press too hard or the cookie will fall apart. Fill with 1/2 teaspoon of jam.
6) Bake for 12-15 minutes or until slightly firm. Allow to cool for a few minutes on the cookie sheet to firm up before moving them to a wire rack to finish cooling.
Makes 2 dozen cookies.