Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper or a silicone baking mat.
Using a stand mixer or hand mixer, beat the softened butter and granulated sugar on medium speed until smooth and creamy.
Add the egg yolk and vanilla extract, and continue mixing until fully combined
Add all-purpose flour and salt into the butter mixture. Mix until a soft dough forms.
Roll the dough into small balls, about 1 inch in diameter then roll into a bowl of sugar. Place the balls on the prepared baking sheet, leaving some space between each cookie.
Gently press your thumb into the center of each cookie to create a heart shape. Be careful not to press all the way through the dough.
Spoon a small amount of strawberry jam (or your chosen jam) into the heart shaped cavity of each cookie. Do not overfill, as the jam may spread while baking.
Bake the cookies in the preheated oven for 10-12 minutes, or until the edges of the cookies are lightly brown.
Let the cookies cool on the baking sheet for a few minutes, then transfer them to a wire rack to cool completely. Share and enjoy!