Preheat the oven: Preheat to 350°F. Line two baking sheets with parchment paper.
Cream the butter and sugar: In a large mixing bowl, beat the softened butter and granulated sugar together for 2–3 minutes, until light and fluffy.
Add egg yolk and vanilla: Mix until fully incorporated and smooth.
Add dry ingredients: Stir in the flour and salt just until a soft dough forms. Do not overmix.
Scoop the dough: Portion into 1-tablespoon balls and place about 2 inches apart on the prepared baking sheets.
Shape the hearts: Press two small indents side by side near the top of each dough ball. Then press one slightly deeper indent below to form a heart shape. Gently adjust with your fingers as needed.
Fill with jam: Spoon about ½ teaspoon of strawberry jam into each center.
Bake: Bake for 12–14 minutes, until the edges are set and the bottoms are lightly golden.
Cool: Let cookies cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.