Preheat the oven to 180°C/350°F/gas mark 4.
*First, make the flax egg. Get out a small ramekin and add the flax seed and water. Mix until the flax seed is pretty much covered with the water. Let sit until needed.
Whisk together the flour, baking soda and salt in a medium-sized bowl. Set aside. In a larger bowl, cream together the butter and sugars until light and fluffy. Add in the flax egg and vanilla, mixing until combined.
In ⅓ batches, add in the flour and stir until no dry flour pockets are left before adding in the next ⅓. Halfway through mixing the last ⅓ of flour, pour in half of your chocolate, reserving the other half for adding on top before baking the cookie.
If you have the time, put the cookie dough in the refrigerator for 2 hours. This step is totally optional, but it makes the dough easier to handle.
Measure the dough out in rounded tablespoons and drop onto an ungreased baking tray about 2 inches apart. Now top the dough with your reserved chocolate. Pop into the oven for 8-11 minutes. 8 minutes if you like gooey cookies and up to 11 minutes if you like a crunchier cookie.
Let cool for a while and they’re ready for munching! Serve with a cold glass of dairy-free milk.