Prep the oven and baking sheets. Preheat the oven to 350°F. Position two racks evenly spaced in the oven. Then, line two large baking sheets with parchment paper and set aside.
Cream the butter and sugar. In the bowl of a stand mixer fitted with the paddle attachment (or using a handheld mixer in a large bowl), cream the softened butter and sugar on medium speed until light and fluffy, about 2-3 minutes. Then, add the egg yolks and vanilla extract and beat for 1-2 minutes until the mixture is pale and creamy.
Add the dry ingredients. In a separate small mixing bowl, whisk together the flour and salt. Gradually add the dry ingredients to the butter-sugar mixture, one cup at a time, mixing at a low speed after each addition. Mix just until combined—avoid overmixing.
Chill the dough. Cover the bowl tightly with plastic wrap and refrigerate the dough for 30 minutes to firm up. This step helps the cookies maintain their shape while baking.
Roll into balls and coat with sugar. Pair the chilled dough with a cookie scoop or tablespoon and roll into 1-inch balls. Then, roll each ball in the additional granulated sugar, ensuring an even coating. Place the balls on the prepared baking sheets, spacing them 1 inch apart.
Make indentations and add jam. Using your thumb or the handle of a wooden spoon, gently press into the center of each dough ball to create a well. Be careful not to press all the way through.
Add the jam. Fill each indentation with approximately ¼ teaspoon of jam using a small spoon or a piping bag for precision. Avoid overfilling, as the jam can overflow during baking.
Bake the cookies. Bake the cookies in the oven for 12-14 minutes or until the edges and bottoms are lightly golden brown. The jam will be bubbly, and the cookies will appear set.
Let it cool. Remove from the oven and let the cookies cool on the baking sheets for 5 minutes before transferring to a wire rack to cool completely. Be cautious—the jam filling will be hot from the oven.