Preheat oven to 350°F. Line a cookie sheet with a silicone baking mat or parchment paper.
Prepare the flax egg and set it aside.
Combine butter and sugars in a mixing bowl and mix on medium to medium-high speed until light and fluffy. Add the flax egg and mix on medium speed until everything is combined.
In a separate bowl, sift together the flour, cornstarch, baking soda, baking powder, and salt.
Pour the flour mixture in with the butter mixture and mix on low to combine. Add the chocolate chips and dried blueberries and stir until they’re evenly distributed throughout the dough. This dough can dry out quickly so be sure to cover it. Be prepared to add a tablespoon or so of water if it dries out.
Drop by rounded tablespoonful onto an ungreased cookie sheet, allowing at least two inches between each. Bake for 9 minutes or until the cookies get a golden color around the edges.
Remove from the oven and allow them to cool for a minute before transferring to a wire rack to cool.
Repeat until all the cookies are done.
I find baking the cookies for 9 minutes works best because when you leave them on the pan, they will continue cooking. That means they're quite soft coming out of the oven, but firm up as they cool off.