Preheat the oven to 350°F (177°C) and 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 and sputter a bit. Cook the butter, swirling the pan often, until the butter smells nutty and has brown bits at the bottom, about 5 minutes. Pour the butter into a medium mixing bowl and let it cool slightly.

Then, add the brown sugar. Whisk the mixture for 1-2 minutes - it 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, baking soda, and salt. Using a rubber spatula, fold the flour mixture into the butter-sugar mixture until it's well-combined and there are no more streaks of flour.

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

Bake the cookies for 7-8 minutes. Then, use oven mitts to grip the sides of the pan and tap it firmly against a hard, heatproof surface to get those wrinkly edges. Return the pan to the oven and bake the cookies for another 1-2 minutes, until the edges are set and the tops are a bit underdone and gooey.

Let the cookies cool on the sheet pan for about 5 minutes. Then, transfer them to a wire rack to cool completely. Enjoy!