In a medium bowl, whisk flour, salt, corn starch, and baking soda together.
In the bowl of a stand mixer fitted with a paddle attachment, cream butter, brown sugar, and granulated sugar together until creamy, about 2 minutes. Add egg and vanilla extract.
Slowly add the dry ingredients. (I added mine in 3 separate parts). Dough will be quite thick.
With a rubber spatula, fold in chocolate chunks and walnuts.
Roll dough into 1 or 1.5 inch balls and place on cookie sheet.
Bake for 10-12 minutes or until cookies are slightly golden brown. The center should look a little undercooked. (This is ok, as they will firm up while cooling.)