01 - Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper.
02 - In a small bowl, combine the ground flaxseed with water and let it sit for 5 minutes to thicken into a gel-like consistency.
03 - In a large mixing bowl, beat the softened vegan butter and granulated sugar together until light and fluffy. Add the flax egg and vanilla extract, then mix until fully incorporated.
04 - In a separate bowl, whisk together the all-purpose flour, baking soda, baking powder, and salt until evenly distributed.
05 - Gradually add the dry ingredient mixture to the wet ingredients, stirring until just combined. Do not overmix.
06 - Gently fold the blackberries into the dough, taking care not to crush them and spread their color throughout.
07 - Scoop tablespoon-sized portions of dough and roll into balls. Roll each ball in granulated sugar to coat evenly.
08 - Place the dough balls on the prepared baking sheets, spacing them about 2 inches apart to allow for spreading.
09 - Bake for 11 to 13 minutes, until the edges are just set and the centers remain soft and slightly underbaked.
10 - Let the cookies rest on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.