Beat the butter and sugar with an electric mixer for about 3 minutes.
Separate the eggs. Set aside the egg whites in a separate shallow bowl.
Add the yolks and vanilla extract to the butter mixture. Mix well.
Add the flour and salt to the mixing bowl and mix until just combined.
Chill dough, 30 minutes
Preheat the oven to 350F.
Line two baking sheets with parchment paper.
Roll the dough into balls about 1 inch in diameter. If using nuts, whisk the reserved egg whites until frothy. Dip the dough balls into the egg whites and then roll them into the nuts until covered. Arrange them on the baking sheets a few inches apart. Place the balls on the parchment-lined cookie sheets spaced a few inches apart.
Press down on each ball of dough with your thumb to make a small well in the center of the cookie. Fill with 1/2 teaspoon of jam.
Bake for 12 to 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.