Prepare Raspberry Puree: Mash fresh raspberries in a bowl with a fork, adding a sprinkle of sugar. Continue mashing until thick and syrupy, add lemon juice if desired, and set aside.

Mix Dry Ingredients: Whisk together flour, baking soda, and salt until well combined and no clumps remain. Set aside.

Cream Butter and Sugar: Beat unsalted butter and granulated sugar together until light and fluffy, about 3-4 minutes. Add egg and vanilla, mixing until smooth.

Combine Wet and Dry Ingredients: Gradually add dry mixture to the wet mixture, mixing gently until a soft dough forms. Fold in raspberry puree, creating swirls.

Preheat and Bake: Preheat oven to 350°F (175°C). Scoop dough onto lined baking sheets, bake for 10-12 minutes until edges are golden and centers are soft.

Cool and Serve: Allow cookies to cool for about 5 minutes on the sheet, then transfer to a wire rack to cool completely before serving.