Preheat the oven to 350°F (177°C). Line a sheet pan with parchment paper.

Melt the butter in a medium skillet over medium-low heat. Bring it to a gentle simmer. It will foam up a bit. Cook the butter, swirling the pan often, until the butter smells nutty and has brown bits at the bottom, about 5-7 minutes. Pour the butter into a medium mixing bowl and let it cool slightly.

Then, add the brown sugar to the brown butter and whisk until well-combined. The mixture will be grainy. Add the egg yolk and vanilla extract. Whisk until the mixture is thickened and fluffy, another 1-2 minutes.

Add the flour, cornstarch, old fashioned oats, baking soda, and salt. Using a rubber spatula, fold the dry ingredients into the butter-sugar mixture just until the dough is well-combined and there are no more streaks of flour. Don't over-mix!

Using a #20 cookie scoop (3 Tablespoons), divide the dough into 6 large balls of dough. Roll each ball of dough and place the cookies about 3 inches apart on the prepared sheet pan.

Bake the cookies for 8-9 minutes, until the edges are set and the tops are a just bit underdone. Let the cookies cool on the sheet pan for about 5 minutes. Then, transfer them to a wire rack to cool completely. Enjoy!