Make the Raspberry Swirl: In a small bowl, mash fresh raspberries with sugar and optional lemon juice until you achieve a thick, puréed consistency—this should take about 2-3 minutes.

Prepare the Cookie Dough: In a medium bowl, whisk together all-purpose flour, baking soda, and salt. In a large mixing bowl, cream softened unsalted butter and granulated sugar until light and fluffy. Add the large egg and vanilla extract, then gradually add the dry ingredients to create a soft dough.

Add the Raspberry Swirl: Gently fold the prepared raspberry purée into the cookie dough, making sure to keep the swirls visible.

Scoop and Bake: Preheat your oven to 350°F (175°C) and line baking sheets with parchment paper. Scoop the dough into equal-sized balls and place on the prepared sheets. Bake for 10-12 minutes until edges are lightly golden.

Cool and Serve: Let cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.