01 - Preheat oven to 350°F and line two baking sheets with parchment paper.
02 - In a large bowl, beat softened butter and granulated sugar together until mixture appears light and fluffy.
03 - Mix in egg yolk and vanilla extract until fully incorporated into the butter mixture.
04 - Add salt and flour, mixing until just combined. Avoid overmixing to prevent tough cookies.
05 - Scoop tablespoon-sized portions of dough and roll each into a ball. Arrange on prepared baking sheets with approximately 2 inches of space between cookies.
06 - Press your thumb or the back of a small spoon into the center of each dough ball to form a deep indentation.
07 - Spoon approximately 1/2 teaspoon of flower jam into each well, being careful not to overfill.
08 - Bake for 12 to 15 minutes until edges begin turning golden brown.
09 - Let cookies rest on baking sheets for 5 minutes before transferring to a wire rack. Once completely cooled, optionally dust with powdered sugar.