Dissolve the instant coffee powder in the water and keep aside to cool.
Using an electric mixer on medium speed, beat together the butter and powdered sugar till smooth, not light and fluffy (about 3 minutes). Beat in the vanilla and dissolved coffee.
Turn down the mixer to low speed and add the flour and salt, mix only till the flour is incorporated into the dough. Overworking the dough is not a good idea as it affects the texture of the shortbread.
Now fold in the chocolate chunks with a spatula or wooden spoon.
Using the spatula, transfer the dough (it will be a bit sticky) into a large Ziploc bag. Leave the bag open, place on your work surface and slowly press out the dough with a rolling pin so it becomes a 9" by 10 1/2" rectangle about 1/4" thick.
Turn the bag to the other side as well, while rolling, and occasionally lift the bag from the dough to smooth out the creases. Take care to see no air pockets remain and gently press out any. Seal the Ziploc bag and refrigerate for at least two hours or upto 2 days.
The longer the dough is chilled the easier the dough is to handle, and the better the cookies hold their shape when baked. So please make sure the dough chills for about 24 hours.
I got some not-so-perfect looking cookies the first time I made them as I chilled the dough only for 2 hours. I needed to make these cookies at a bit of a short notice and couldn't chill them any longer.
My daughter wanted these as her after school snack with milk; I also wanted to send some with my husband who was traveling out for a few days and gift some to a blogging foodie who had come down to Goa.
The next time I made them, I chilled them overnight. When the dough has chilled enough, take it out. Place on a cutting surface and slit the Ziploc bag open. Take the chilled rectangle of dough out, and cut into 1 1/2" squares with a sharp knife after marking them out using a ruler.
Place the squares on a baking sheet lined with parchment (I just placed them on ungreased cookie sheets) and prick them well with a fork (I forgot to do this, but it didn't affect my cookies much)
Bake at 180C (350F) for 18 to 20 minutes till they look golden. The baking time needs to be judged well, as the dough is already a pale brown, or one might end up with slightly burnt shortbread squares that no one wants to eat! So keep a watch on them once they've baked for 15 minutes.
Dust the shortbread squares with powdered sugar while they're hot, if you prefer. Cool on a rack. I forgot to count how many cookies I got, but there were quite a few. If you like coffee and chocolate these shortbread squares are most definitely for you.