Preheat oven to 350 degrees F and prepare a 9x9 inch square cake pan with non stick spray and lined with parchment paper
In a medium bowl combine the flour, baking soda, cornstarch and salt; set aside
In a mixing bowl with paddle attachment, combine the melted butter and light brown sugar; mix on medium speed until creamy and fully combined
Add the egg, egg yolk and vanilla extract to the butter mixture and continue mixing until combined
Add the flour mixture into the butter mixture and mix on medium speed until the dough comes together
Fold the semisweet and milk chocolate chips into the dough
Press the blondie dough evenly into the prepared square pan
Bake for 20-22 minutes or until the top is a light golden brown; remove the blondies from the oven and slightly sprinkle the top with course sea salt for a salty sweet finish!
Allow the blondies to cool completely before removing them from the pan, approximately 30-45 minutes (refrigerate the blondies if you need them to cool faster)
Remove the blondies from the pan and cut them into 16 square pieces