Preheat oven to 350 degrees Fahrenheit. Cover a baking sheet with parchment paper.
Using an electric mixer on medium-high speed, combine the butter and sugar in a large mixing bowl until the mixture becomes light and fluffy (about 2 minutes).
Mix in the egg yolks and vanilla extract.
Next, mix in the flour and salt. Blend until the dough comes together into a ball.
Roll the dough into 1-inch sized round balls. Place them onto the prepared cookie sheet.
To create the heart-shaped centers, use your thumbs to create each half of the heart, angling the bottom of your thumb toward the center. Alternatively, use the top of a dull butter knife to create the tops of of the heart, angling the bottom of the knife toward the center before rotating the knife to create the other heart top.
Carefully place enough strawberry jam into each newly created heart-shaped divot. Use just enough jam to fill the crater without it overflowing.
Place the baking sheet into the preheated oven and bake for 18-20 minutes. Remove the cookies from the oven when the shortbread holds it shape when it is touched, but is still soft.
Leave the cookies on the baking sheet for at least 5 minutes. Move the cookies to a wire rack to continue cooling.