Preheat oven to 375 degrees F. Line a baking sheet with parchment paper or a silicone mat.
With an electric mixer on medium-high speed, beat together butter and sugar until light and fluffy, 2-3 minutes. Beat in egg yolks and vanilla extract. Mix in flour and salt until incorporated and dough comes together in a ball.
Form dough into 1-inch balls and arrange on prepared baking sheets. Flatten balls slightly with your thumb or the back of a small spoon, leaving an indentation in the center.
Bake cookies for 8 to 10 minutes, or until bottoms are just barely golden. Remove baking sheet from oven. If indentations look shallow, further define them with the back of a spoon and then fill each with approximately 1/2 teaspoon jam.
Bake for an additional 3 to 4 minutes, or until jam melts slightly and edges of cookies are lightly golden. Jam should be sticky and soft. Transfer cookies to wire racks to cool.