Preheat the oven to 360°F (182°C). Line two sheet pans with parchment and set aside.
In a medium bowl, whisk together the flour, cornstarch, baking soda, fine sea salt, and cinnamon until evenly combined.
In a stand mixer fitted with the paddle (or using a hand mixer), beat the softened butter with the brown sugar and granulated sugar on medium-high speed until creamy and slightly lightened, about 2 minutes. Scrape down the bowl.
Beat in the eggs one at a time, mixing just until incorporated after each addition. Mix in the vanilla until the mixture looks glossy and smooth.
Add the dry ingredients in two additions on low speed, stopping when a few flour streaks remain. Fold in the rolled oats and 1 1/4 cups of the chocolate chips (reserve the rest for topping).
Let the dough rest for 15 minutes at room temperature to hydrate the oats for extra chew. Meanwhile, scoop 2-tablespoon portions onto the prepared sheets, spacing about 2 inches apart, and press a few reserved chips on top of each mound. If your kitchen is warm, refrigerate the filled trays for 10 minutes before baking to control spread.
Bake one sheet at a time for 10 to 12 minutes, until the edges are set and lightly golden but the centers still look slightly soft. Tap the sheet once on the counter right out of the oven to help the cookies settle and crinkle.
Cool on the pan for 10 minutes, then move cookies to a wire rack to finish cooling. Repeat with the remaining tray.