Preheat the oven to 350 degrees F and line a 8 or 9 inch square baking pan with parchment paper, leaving overhang on the sides for easy removal.
In a small bowl, stir the ground flaxseeds and water and let it sit for a few minutes. This is your flax egg.
In a large bowl, whisk the melted vegan butter with the brown and granulated sugars until smooth. Now add in the flax egg and vanilla and stir with a large spoon to combine.
To the bowl with the wet butter/sugar mixture, add the flour, cornstarch, baking soda, salt and stir until combined. The dough should be thick and soft.
Fold in almost all of the chocolate chips, saving some for the top.
Transfer to the prepared pan and press it down evenly. Sprinkle with more chocolate chips if desired, and bake for 30-32 minutes until lightly golden on top.
Let cool in the pan for 15-20 minutes, then lift the parchment paper out of the pan and cut into squares. Will keep in an airtight container for about a week. Can be frozen as well. Enjoy!