Prepare the Raspberry Filling:

In a small saucepan over medium heat, stir together raspberry preserves, cornstarch, and lemon juice.

Simmer for 2-3 minutes, stirring constantly until slightly thickened.

Remove from heat and let it cool completely.

Make the Shortbread Dough:

In a large bowl, cream together softened butter and granulated sugar until light and fluffy.

Mix in the egg yolk and vanilla extract.

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

Divide the dough in half and shape each half into a rectangle. Wrap in plastic wrap and refrigerate for 30 minutes.

Assemble the Cookies:

Roll out one dough portion into a ¼-inch thick rectangle on a floured surface.

Spread half of the cooled raspberry filling evenly over the dough, leaving a ½-inch border around the edges.

Carefully roll the dough into a log, pressing gently to seal. Repeat with the second portion.

Wrap each log in plastic wrap and refrigerate for 1 hour (or freeze for 20-30 minutes for easier slicing).

Bake the Cookies:

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

Slice the chilled dough logs into ¼-inch thick cookies and place them on the baking sheet 1 inch apart.

Bake for 12-15 minutes, or until the edges turn lightly golden.

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

Make the Glaze (Optional):

In a small bowl, whisk together powdered sugar, milk (or lemon juice), and vanilla extract until smooth.

Drizzle over the cooled cookies for an extra touch of sweetness.