Make the cookie dough:

1. Beat the butter and sugar on high speed with an electric mixer for about 3 minutes.
2. Separate the eggs. Add the yolks and vanilla extract to the butter mixture. Set aside the egg whites in a separate shallow bowl.
3. Add the flour and salt to the mixing bowl and mix until just combined.

Chill the dough:

• Place the dough in the fridge and chill for at least 30 minutes.
• Preheat the oven:
  Preheat the oven to 350°F. Line two baking sheets with parchment paper.

Shape the cookies:
Roll the dough into balls about 1 inch in diameter. Place the balls on the parchment-lined cookie sheets spaced a few inches apart.
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.

Thumbprint the dough, and add the jam:

Press down on each ball of dough 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 each cookie with 1/2 teaspoon of jam.

Bake, then let cool:

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. Serve warm or at room temperature.