Preheat your oven to 350°F (180°C) and prepare your baking sheets by lining them with parchment paper.

If using frozen blackberries, thaw them completely and puree until smooth, yielding about ½ cup of blackberry puree.

In a large mixing bowl, beat softened butter and granulated sugar together until light and fluffy, around 2-3 minutes. Gently fold in cooled blackberry puree and vanilla extract.

In a separate bowl, whisk together flour, baking soda, and salt. Gradually add the dry mixture to the wet ingredients, stirring until just combined.

Fold in chocolate chips (or nuts) into the cookie dough. Chill the dough in the refrigerator for at least 30 minutes.

Scoop tablespoon-sized balls of dough, flatten slightly, and place on baking sheets. Bake for 8-10 minutes until edges are set.

Allow to cool on the baking sheet for a few minutes, then transfer to a wire rack to cool completely.