Set oven to 350F. Line an 9x9 inch baking sheet with parchment paper and melt the butter you'll need for the recipe in a large bowl.
Add the flour, salt, baking soda, and corn starch to a medium bowl then whisk together and set aside.
Pour the melted butter into a large bowl. Add the white and brown sugars and whisk together, they'll form a somewhat thick slurry.
Add the eggs and vanilla to the butter mixture and mix together well.
Dump the dry mixture into the wet and mix until almost combined then stir in all but a handful of the white chocolate chips, mixing so they're distributed.
Transfer the dough to an 9x9 inch baking dish lined with parchment paper and spread toward the edges giving it a rough smooth. Top with the reserved white chocolate chips.
Bake for about 35 minutes at 350F. The cookie bars are done when the edges are set and golden brown and a toothpick or skewer inserted into the center comes out mostly clean. Let the cookie bars cool in the pan for about an hour before removing and cutting.
It is COMPLETELY essential to line your baking dish. If you don't have parchment paper you can use foil instead but don't try to skip this or get away with butter and flouring the pan.
Add a cup of walnuts or pecans for a nice crunch.
A sprinkle of flaked sea salt on top just after baking provides a nice bit of contrast.
Love that gooey brownie texture? Try under-baking this a bit and enjoy!