Preheat oven to 350℉. Line a baking sheet with parchment paper.
To a stand mixer fitted with a paddle attachment add the butter, brown sugar and granulated sugar. Cream together on medium low until light and fluffy, about 3-4 minutes. Scrape down the sides as needed.
Next with the mixer on low add the eggs one at a time then the vanilla. Scrape down the sides.
In a medium bowl whisk together the flour, cornstarch, baking soda, baking powder and sea salt. With the mixer on low, slowly add in the flour mixture and mix until you no longer see any flour bits, about 1 minute. Add in the oats and mix for 30 seconds.
Stir in the chocolate chips and walnuts by hand.
If you have the time, scoop the dough out onto plastic wrap and tightly wrap up. Place in the fridge overnight up to 48 hours. If not, continue scooping right away.
Using a large, 3 tbsp. ice cream or portion scoop, scoop the dough onto the lined baking sheet leaving a 2-inch border around the cookies. Sprinkle with flaked sea salt. If scooping chilled dough lightly press down to flatten before sprinkling with salt.
Bake for 13-15 minutes or until set and golden around the edges. Let cool slightly on the sheet tray before removing to a cooling rack and letting cool fully. Continue with remaining dough. Makes 30 cookies.