Start by thawing frozen blackberries if you're using them. Then, puree the berries with an immersion blender until smooth, ensuring you have about ½ cup of blackberry puree. Set aside to cool.

In a medium bowl, use an electric mixer to cream together softened butter and granulated sugar until the mixture is light and fluffy. Next, add in your cooled blackberry puree and vanilla extract, mixing until everything is well combined.

In a separate bowl, whisk together the all-purpose flour, baking soda, and salt. Gradually add this dry mixture to your wet ingredients, stirring until just combined. Gently fold in the chocolate chips, reserving about ¼ for topping.

Cover your cookie dough and chill it in the refrigerator for at least 30 minutes.

Preheat your oven to 350°F (180°C). Roll tablespoon-sized portions of the chilled dough into balls and place them onto a lined cookie sheet.

Bake your cookies for 8-10 minutes, or until the edges are set and lightly golden. Sprinkle the reserved chocolate chips on top immediately after baking.

Let the cookies cool on the tray for several minutes before gently transferring them to a wire rack to cool completely.