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

Pour the melted, cooled butter and dark brown sugar into a medium mixing bowl. Whisk until the mixture is thickened and lighter in color, 2-3 minutes.

Add the egg yolk and vanilla extract. Then, stir in the creamy peanut butter. Using a rubber spatula, fold the flour, cornstarch, baking soda, and salt into the peanut butter mixture, just until the dough comes together and there are no more streaks of flour. Don't over-mix!

Using a #20 cookie scoop, divide the dough into 6 big balls. Roll the cookie dough balls between your palms until they're smooth and then roll them in the granulated sugar.

Place the cookie dough balls 2-3 inches apart on the prepared sheet pan. Bake the cookies for 5 minutes. Then, using oven mitts, grip the sides of the pan and "bang" the pan on the wire rack a few times, until the cookies crinkle on the edges. Bake for 4-5 more minutes, just until they're set on the edges and a little under-done in the centers.

Cool the cookies on the sheet pan for 5-10 minutes. Then, transfer them to a wire rack to cool completely. Enjoy!