Preheat the oven to 350 F and prepare a baking sheet with parchment paper. Place the flax and water into a small bowl and stir together and then set it aside to gel while you prepare your other ingredients.
In a mixing bowl, use a hand held mixer to cream together the vegan butter and sugars. Add in the vanilla and mix until combined. Add in the flax mixture and continue beating until well blended. Add in the flour, cocoa powder, espresso, baking soda and salt to the mixture and beat until no dry spots remain. Fold in the chocolate chips and hazelnuts. The cookie dough will be a little wet and that's okay, do your best to form it into a large ball and place it in the center of the baking tray.
Bake the cookie for 8 minutes, remove it from the oven and tap (I like to drop it from 4-5 inches onto the counter) the baking sheet on the counter 3-4 times to help deflate the cookie. Return to the oven for 4-5 minutes and golden brown. The center may look undone but it will set as it cools. Let the cookie cool for at least 15 minutes before serving