Preheat oven to 350°F (177°C). Line two large baking sheets with parchment paper or silicone baking mats and set aside.
In the bowl of a stand mixer fitted with the paddle attachment, or in a large mixing bowl using a handheld mixer, cream together the butter and sugar for 1-2 minutes or until well combined. Mix in the eggs yolks, vanilla extract, and salt until fully combined, then mix in the flour. The mixture will be a little crumbly at first but it will come together as you continue mixing it.
Using a one-tablespoon cookie scoop or measuring tablespoon, measure out the cookie dough, roll into balls, roll each one in granulated sugar (optional), then place on the prepared baking sheets.
Use your thumb (or the handle of a large wooden spoon) to press an indentation into each ball of cookie dough. Spoon 1/2 teaspoon of jam into the indentation in each one.
Bake at 350°F (177°C) for 12-14 minutes or until the cookies are set and the bottoms are lightly browned. Remove from the oven and cool on the baking sheet for 10 minutes, then carefully transfer the cookies to a wire rack to cool completely.