Preheat your oven to 375°F (190°C) and line a baking sheet with parchment paper.

In a mixing bowl, cream together softened butter and brown sugar. Beat in egg or flax egg and vanilla extract until smooth.

Sift in all-purpose flour and salt, mixing on low speed until just combined to form a soft dough.

Roll dough into 1-inch balls and place them on the baking sheet with space in between.

Create an indentation in each ball with your thumb, shaping it into a heart.

Bake for 12-15 minutes or until edges are lightly golden; redefine heart shapes if necessary.

Cool cookies on a baking sheet for a minute before transferring to a wire rack to cool completely.

Once cooled, fill each heart-shaped indent with your choice of jam.