Prepare Raspberry Puree: Wash and mash fresh raspberries, adding granulated sugar and lemon juice if desired, creating a thick puree; set aside.

Mix Dry Ingredients: Whisk together all-purpose flour, baking soda, and salt in a separate bowl; set aside.

Cream Butter and Sugar: In a mixing bowl, cream together softened butter and granulated sugar until light and fluffy; add egg and vanilla extract, mixing until smooth.

Combine Ingredients: Gradually stir the dry flour mixture into the butter and sugar mixture until a soft dough forms.

Swirl in Raspberry Puree: Gently fold the raspberry puree into the dough, creating a marbled effect; avoid overmixing.

Bake Cookies: Preheat oven to 350°F (175°C); drop rounded balls of dough onto prepared sheets, spacing 2 inches apart; bake for 10-12 minutes.

Cool: Let the cookies cool on the sheets for about 5 minutes before transferring to wire racks to cool completely.