Preheat your oven to 350°F (180°C). Line a baking tray with silpat or parchment paper. In a small bowl, mix the flaxseed and 3 tablespoons of water and set aside for 2-3 minutes to thicken. This will be your flax egg/binder.
Meanwhile, combine the flour, cocoa powder, sugar, baking powder and salt in a large bowl. Add in the rest of the ingredients ending with the flax egg. Once the dough starts to get too thick to mix with a spoon switch to mixing with your hands until it comes together in a ball. If the dough looks too dry, you can add 1 tablespoon of melted butter and gently smoosh to incorporate it into the dough.
Transfer the cookie dough to a clean surface, shape roughly into a rectangle and cover with cling wrap. Chill in the fridge for at least 15 minutes or up to overnight.
When ready to roll, grab 2 separate parchment paper sheets large enough to span your ideal work surface. Lay one sheet down and place the dough on top. Cover the dough with the other sheet and start rolling it out until it is ⅙ inch thick, peeling off the parchment paper every now and then to check on the evenness. Then, use your favorite cookie cutter to cut out shapes.
Transfer the cookies to the prepared pan and bake for 8-10 minutes or until the cookie appears matte and the smell of chocolate wafts through the room. Set aside until fully cooled.