line a 9 by 13 inch baking pan with parchment paper and lightly grease.
Make Batter
whisk flour baking soda cornstarch and salt
stir in oats
in a separate bowl whisk melted butter brown sugar and granulated sugar until smooth
whisk in eggs and vanilla
fold dry ingredients into wet ingredients just until combined
fold in chocolate chips and walnuts if using.
Bake and Cool
spread batter evenly in pan and press lightly
bake until edges are golden and center is set 20 to 25 minutes
cool completely in pan before lifting out and slicing into bars.
Notes
1. For thicker bars use a 9 by 9 inch pan and bake 25 to 30 minutes. Swap walnuts for pecans or omit nuts. Use dark chocolate chips or a mix of chips and chopped chocolate. Do not overbake for the chewiest texture