Preheat oven to 375 degrees C. Line 2 baking sheets with parchment paper.
In a small bowl, mix two tablespoons of ground flax with two tablespoons of water and set aside.
Sift flour with cocoa powder, baking soda, cayenne and cinnamon in a large bowl. Stir in chocolate chips.
In a stand mixer or bowl, cream flax egg with both sugars, oil, ginger juice and vanilla. Stir into flour mixture until combined.
Using a 1/4 cup measure, scoop 6 portions of dough onto each baking sheet. Flatten cookies slightly with the bottom of the measuring cup. Sprinkle dough with salt or sugar.
Bake 1 pan at a time in centre of oven until cookies start to crack, 11 to 13 min. They should still be fudgy in the middle. Cool on racks.