Mash the raspberries with sugar and lemon juice (if using) in a small bowl until it forms a thick puree. Set it aside to incorporate into the dough later.

In a separate bowl, whisk together the flour, baking soda, and salt. In another mixing bowl, beat the softened butter and sugar together until light and fluffy. Add the egg and vanilla extract and continue beating until well combined.

Gradually add the dry ingredients to the wet mixture and mix until the dough just forms.

Gently fold the raspberry puree into the dough using a spatula. Be careful not to overmix; you want to maintain streaks to create a marbled, vibrant effect in the cookie dough.

Preheat your oven to 350°F (175°C). Line baking sheets with parchment paper. Scoop tablespoon-sized balls of dough and place them spaced a few inches apart on the baking sheets.

Bake for 10–12 minutes, or until the edges start to turn golden.

Allow the cookies to cool on the baking sheets for about 5 minutes before transferring them to a wire rack to cool completely or serve warm.