Shortbread with a snap
Your recipe is a bit short of flour for a classic 3:2:1 flour:butter:sugar ratio, counting both the flour and cornstarch with the flour. You have 1298 grams of flour and cornstarch combined, it needs to be closer to 1386. A bit more flour will certainly help, but your method should also be considered. I would recommend cutting the butter in rather than mixing in order to deliver the best crunch. 

Last, if you're cutting them into fingers then it sounds like it's pretty thick. I've found it's hard to get thick shortbread to be crunchy without overcooking it, I'd suggest thinner is better. 

When baking use the touch test to detect when it's ready, you want to bake it until it firms up some, a few test batches will give you an idea when it's reached the point.