Make a flax 'egg' by mixing the ground flax seed with water. Set aside to become gloopy.
In a large bowl, whisk together the butter and the sugar until it becomes fluffy. By hand this will take around 5 mins, with an electric whisk, approx 2, depending on its speed.
Whisk in the flax 'egg' for about 15 seconds, to make sure it's fully incorporated.
Add the flour, baking soda, and salt, and whisk together until you have a crumbly dough.
With your hands, bring the dough together to form a ball. It will be very soft.
Gently knead in the chocolate.
Divide and roll into 12 balls, and place onto the cookie sheet, making sure they're not too close together. There's no need to flatten the balls.
Bake for 10-12 minutes, until the cookies have flattened and spread, and are just becoming golden. They will be very soft at this point but will firm up as they cool.
Remove from the oven, and set aside to cool on the cookie sheet.
Once cooled, keep in an airtight container for up to a week.... as though they stand any chance of being around that long!
Or use ground chia seeds.
Depending on the type of flour you use, you may find that the dough is a bit too sticky - if that's the case, add a little more flour.
If you don't have baking soda, use ½ tsp baking powder and ¼ tsp bicarbonate of soda.
Chocolate: I've made these cookies with dark and milk chocolate (all vegan, of course) - both work equally well, so use your favourite. Unless you're a die-hard bitter chocolate fan though, anything above 65% cocoa solids may be a bit intense!