Preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper.

Cream together the softened butter and granulated sugar in a mixing bowl until light and fluffy, about 2-3 minutes.

Mix in the egg yolk, lemon zest, lemon juice, and crushed lavender until well combined.

Gradually add the flour and salt, mixing just until a cohesive dough forms.

Roll small portions of dough into balls and press your thumb in the center to create the thumbprint.

Bake for 10-12 minutes, or until the edges are lightly golden.

Allow cookies to cool for 5 minutes on the baking sheet, then transfer to a wire rack.

Prepare the lemon glaze by mixing powdered sugar with lemon juice until smooth, then fill each thumbprint with the glaze.