Preheat oven to 350 degrees. Lightly spray a 9x13 inch baking dish with cooking spray. Or you can line with aluminum foil and spray with cooking spray for easy removal. In a medium mixing bowl, whisk together the cake flour and cornstarch, set aside.
Using a mixer, cream together butter, sugar and brown sugar. Add beaten egg yolk, vanilla, and cake flour cornstarch mix until all incorporated.
Pat the dough into the bottom of your 9x13 inch pan. Bake for 15-20 minutes until the edges lightly brown. Let cool slightly.
In a microwave safe dish or using a double boiler, melt the milk chocolate. I melted it in the microwave stirring 30 seconds at a time. Pour the chocolate on top of the cookie crust and spread evenly. Sprinkle the nuts on top.
Cut into small squares once cooled, and store in airtight container.