Preheat your oven to 350°F (175°C) and line several baking sheets with parchment paper.
To a high-speed blender add the prunes, cocoa powder, flaxseeds and hot water (in that order). Blend on low for several minutes or until very smooth. Scrape down the sides and bottom of the blender to make sure that everything is well combined. Measure out about ½ cup (170 g) of the prune puree and reserve the remaining for your next batch (it freezes great!). Don’t clean your blender and set aside for the next step.
To a large, heat-safe bowl set over simmering water, add the dark chocolate, oil, brown sugar, granulated sugar and ½ cup (170 g) of prune puree from above. Make sure the water doesn’t boil or touch the bottom of the bowl. Stir frequently until the chocolate is melted and then pour the mixture back into the blender, add the vanilla and espresso/coffee and blend on low until quite smooth, no more than 1 minute.
In a large bowl, whisk together the flour, sea salt, baking powder and baking soda. Scoop the chocolate mixture into the flour mixture and fold until the dough is mixed, but you still see streaks of flour. It will be very thick. Add the chocolate chips and fold until evenly mixed throughout the dough. The dough will appear slightly glossy and quite thick, this is perfect.
Gently roll 2 tablespoons (28 g) of dough into a ball and place it on the cookie sheets, allowing about 2 inches (5 cm) between each dough ball. Optional: If you want the cookies coated in sugar, add the demerara sugar to a shallow bowl and roll each dough ball in the sugar and set on the baking sheet. Bake one sheet at a time in the center of the oven for 10 minutes. You want to slightly underbake these. Allow the cookies to cool on the sheet for 15 minutes then transfer to a cooling rack.