Preheat the oven to 325. Lightly grease a 10-inch cast iron skillet with butter or non-stick spray.Note, if you don't have an oven-proof or cast iron skillet, you can bake these cookie bars in a deep dish pie pan or a square baking pan. If your pan is smaller, the bars will be thicker and may need a couple extra minutes of bake time. If your pan is bigger, the bars will be thicker and will need a couple minutes less of bake time.
In a bowl, use a wooden spoon to stir together the warm melted butter, granulated sugar, brown sugar and vanilla until it smooths out. Beat in the egg and egg yolk, for about a minute, until smooth and ribbony.
Separately, whisk together the flour, corn starch, baking soda, salt and espresso powder. Stir the dry ingredients into the butter mixture just until combined. Let the dough cool for 10 minutes.
Stir 1 1/4 cups of the chocolate chips into the dough; it will be thicker now after cooling.
Press the dough evenly into the greased pan, and sprinkle the top of the dough with the remaining 1/4 cup chocolate chips.
Bake the cookie on the center oven rack for 28-30 minutes. The edges should be puffed, and the cookie golden brown, but still a bit underdone in the center. Sprinkle the top with 1/4 teaspoon flaky salt.
Let the cookie cool for 10-15 minutes and serve warm and gooey with ice cream, or let it cool for about 1 hour to cut it into bars.