Brown the butter: Cook the butter over medium heat, stirring constantly, until it develops an amber color and nutty aroma. It will first melt, then bubble and sizzle, before it browns. This will take a few minutes.
Immediately pour the brown butter into a large heatproof bowl, making sure to scrape the brown bits at the bottom of the pan. Cool for 15 minutes.
Make the cookie dough: In a medium bowl, whisk the flour, oats, cornstarch, salt, baking soda, and cinnamon. Set aside.
Add the water and brown sugar to the cooled brown butter. Whisk until the mixture lightens in color.
Add the egg yolks and vanilla to the butter and sugar mixture. Whisk until combined.
Add the dry ingredients to the wet ingredients and fold until a thick cookie dough forms. Fold in the chocolate chunks. The cookie dough will be slightly sticky.
Cover the cookie dough with plastic wrap and chill for at least 2 hours and up to 2 days.
Bake the cookies: Preheat oven to 350°F (177°C). Line 2 medium or 1 extra-large baking sheet with parchment paper. Set aside.
Portion the chilled dough into 17-18 small cookies, either using your hands or a small cookie scoop. Arrange onto the baking sheet, spacing out each cookie by around 2 inches.
Bake for 10-13 minutes, or until the edges are golden brown and the centers look slightly under done. Pull the cookies out before they look finished because they will continue to cook as they cool.
Allow the cookies to cool on the hot baking sheets for around 10 minutes before serving warm, or transfer the cookies to a wire rack to cool completely.
Notes
Use whole rolled oats, not quick oats.
Use chopped baking chocolate, not chocolate chips. For more info, read the blog post above.