Line a baking sheet with parchment paper and set aside.
In a medium mixing bowl or bowl of a stand mixer, add the softened/room temp butter and both brown and white sugar.
Cream together for about 1 minute. Add in the egg, egg yolks and vanilla. Cream together until combined.
Add in the flour, cornstarch, baking powder, baking soda and cinnamon. Mix together until combined. Be sure to spoon and level the flour to measure.
Fold in the chocolate chips and chopped walnuts. Use a large cookie scoop (mine was about 3 tablespoons) and scoop into 9 cookies. You can certainly make these smaller (and therefore have more), but levain cookies are traditionally quite large.
Scoop onto the baking sheet. Set in the fridge for 1 hour.
Toward the end of the hour, preheat the oven to 375 Fahrenheit.
Arrange the cookies on two baking sheets to leave room for spreading.
Bake for 12-16 minutes or until the edges are golden brown. They will continue to firm up a bit as they set.
Use a round cookie cutter to swivel around the edges and create perfect circles if desired. Sprinkle with flakey salt and enjoy!