Make shortbread dough: In a large bowl, cream together the softened butter and granulated sugar until light and fluffy.

Add flour and salt: Gradually add the flour and salt to the butter mixture, mixing until just combined. Do not overmix. The dough will be crumbly at first but will come together as you mix.

Chill dough: Wrap the dough in plastic wrap and flatten it into a disc. Refrigerate for at least 2 hours, or preferably overnight. This chilling is crucial for preventing spreading and creating a tender cookie.

Prepare raspberry swirl: In a small bowl, combine the raspberry jam and water. Mix well until smooth.

Preheat oven: Preheat oven to 350°F (175°C). Line baking sheets with parchment paper.

Assemble cookies: On a lightly floured surface, roll out the chilled dough to about ¼ inch thickness.

Create swirl: Spread the raspberry jam mixture evenly over the dough. You can use a small offset spatula or the back of a spoon.

Roll and slice: Starting from one edge, roll the dough tightly into a log. Wrap the log in plastic wrap and refrigerate for another 30 minutes to firm up. This will make slicing easier.

Slice cookies: Using a sharp knife, slice the log into ¼-inch thick rounds.

Bake: Place the cookies on the prepared baking sheets, leaving some space between each cookie. Bake for 12-15 minutes, or until the edges are lightly golden. The centers will still look pale.

Cool: Let the cookies cool on the baking sheets for a few minutes before transferring them to a wire rack to cool completely.