Take 1 tablespoon of flaxseed powder and mix it with 3 tablespoons water. Keep this aside.
In a large bowl, add room temperature butter, white caster sugar and brown sugar and using an electric beater or whisk, cream the butter for 3 to 4 minutes till light and fluffy.
Once the sugar and butter are incorporated well and creamy, add the flaxseed mixture and beat for 1 minute.
Next add the vanilla essence and mix well.
In a separate bowl, sieve all the dry ingredients - plain flour, baking soda, table salt, cocoa powder to get rid of any lumps.
Now add the sieved dry ingredients to the wet mix and cut and fold and bring it together. Do not use an electric beater or whisk for this step and just use a spatula. Do not over mix.
Dissolve 1 teaspoon of coffee in 1 tablespoon of hot water.
Add the coffee mix to the cookie dough and add 2 tablespoons of choco chips. Mix gently.
Scrape the bowl to bring together the dough. It might not be too stiff at this stage. Refrigerate this cookie dough covered for at least 1 hour.
Once the cookie dough is chilled, it would be bit stiff and set. This is the consistency at which the cookies are to be baked.
Preheat the oven at 180 degree Celsius with bottom and top heating + fan mode on. In the meanwhile line the baking tray with a butter or parchment paper and slightly grease some butter. The cookies are to be baked in two batches (set of 6) as the cookies spread out on the tray during the bake.
Chop the dark chocolate bar into small chunks
Now using an ice-cream scooper, scoop a small chapati sized ball onto the baking tray. You could also use a round serving spoon if you do not have an ice-cream scooper.
Now push some generous amount of chocolate chunks on top of each cookie.
Transfer the baking tray into the pre-heated oven and bake for 11-13 minutes. The cookie would spread out automatically during the bake. The cookie would have set edges however feel soft and bit under-done in the center at this stage, but it would harden on cooling, so do not overbake!
Pull the baking tray out of the oven and immediately sprinkle some coarse kosher salt on top. Let the cookie rest on the baking tray for 4-5 minutes.
Transfer the cookies on a cooling rack. Repeat the baking for the remaining cookies.
Once cooled completely, transfer the cookies to an air tight container and store at room temperature.