Step Mom Fran would make these every Christmas.
Preheat oven to 350°.
Line two large baking sheets with parchment paper or silicone baking mats and set aside.
In a stand mixer bowl or large mixing bowl using a hand mixer, cream together the butter and 2/3 cup sugar until creamy and smooth, about 3 minutes.
Mix in egg yolks, vanilla extract, and salt until fully combined, then mix in the flour.
Using a 1 tablespoon cookie scoop or measuring tablespoon, measure out the cookie dough, roll into balls, roll each ball in granulated sugar (optional), and then place on baking sheet.
Use your thumb to press an indentation into each ball of cookie dough.
Spoon ½ teaspoon of jam into the indention of each cookie.
Bake for 12-14 minutes or until the cookies are set and the bottoms are lightly browned.
Remove from oven and cool on the baking sheet for 10 minutes.
Then carefully transfer cookies to wire rack to cool completely.
Chef's notes: You may use one kind of jam, or mix it up and use different ones. If making for Christmas, the different jams make a colorful plate. Cookies may be stored in an airtight container at room temperature for up to 4 days. These cookies will freeze well for up to 3 months. Thaw to room temperature before serving. The dough will also freeze well up to 3 months. Thaw overnight in the refrigerator. Original recipe credited to Fran Millender Hoover.