Melt the butter gently using your favorite method. Let cool.
In a medium size bowl, combine flour, cornstarch, baking soda and salt. Mix well.
In a mixing bowl, using a wooden spoon (or lowest speed of an electric mixer) mix the melted and cooled butter with the sugars.
Add the egg and stir until blended, then stir in the yolk. Do not whip the egg, just stir it in.
Add the flour mixture and stir to make a soft dough.
Lay a big piece of parchment or plastic wrap on your work space. Take about ¼ of the dough and press it into a very thin slab -- somewhere around ⅛ of an inch, maybe a little thinner. There's no 100% correct way to do this, so just have fun.
Lay some flat chocolate disks or pieces over the thin layer of chocolate, then add a second layer (another ¼ of the dough) over the chocolate. Repeat with more chocolate, more dough and so on, stacking dough and chocolate.
Shape the dough into a big rectangle that's a little over and inch thick. Trim as necessary. Wrap tightly and chill overnight or at least for several hours.
When dough is solid, carve it into squares. The squares should weigh a little over an ounce each for small cookies or 4 oz each for large. Trim as necessary, patching little pieces of dough on each cookie as needed. Keep dough blocks cold while you preheat the oven. I like to throw them in the freezer.