Make the prune puree. Combine ⅔ cup (4 oz.) pitted prunes with ¼ cup hot water in a powerful blender. Pulse to combine, then blend until a smooth, pourable consistency forms. Scrape the sides if necessary and add in 1-2 Tbsp. more hot water if needed to get a nice consistency.
Whisk the flaxseed and water together in a small bowl to make the flax egg. Allow to sit for 5-10 minutes in the fridge. Stir again, it should be thick and goopy.
Combine the flours, cornstarch, baking powder, baking soda, salt and espresso powder in a large bowl. Set aside.
Preheat the oven to 400°F and line 2 baking sheets with parchment paper or silicone baking mats.
Using a stand mixer fitted with the paddle attachment cream together the vegan butter and white sugar for ~2 minutes. Add in the prune puree and combine again, scraping down the sides of the mixing bowl as necessary.
Add in the flax eggs and vanilla until combined. Gradually add in the dry ingredients, followed by the vegan chocolate chips. Mix just until combined.
Scoop the dough with a cookie scoop onto the prepared baking sheet and gently press down to flatten slightly. Garnish the tops of the cookies with a few more chocolate chips. Be sure to leave about 2 inches of space between the cookies (recipe will make ~12 cookies).
Bake for 9-10 minutes (baking time can vary but I always bake these cookies for 9 minutes in our oven). Cool on the cookie sheet for 5 minutes then transfer to a cooling rack. Garnish with flaky sea salt, if desired and enjoy!