Cut the butter into cubes and place in a heavy-bottomed medium saucepan. Cook until the butter melts and begins to foam. Stir constantly. Remove from heat immediately as soon as the flecks in the pan turn brown and it smells nutty. Set aside to cool completely to room temperature.
Measure out the flour, cornstarch, baking powder, and salt together in a medium bowl. Stir to combine and set aside.
Add the cooled butter to the bowl of a stand mixer, or a large mixing bowl along with the granulated sugar, brown sugar, and vanilla extract. Beat on medium high speed using the paddle attachment of your stand mixer or a hand mixer until light and fluffy - about 3 - 4 minutes.
Add in the egg and beat for an additional 1 - 2 minutes to fully incorporate. Scrape the sides and bottom of the bowl as needed.
Stir in the flour mixture on low, or by hand using a rubber spatula. Mix only until there are no streaks of flour. Scrape the sides and bottom of the bowl as needed.
Cover the bowl and leave the dough to chill for at least an hour, preferably overnight for the best flavor development.
Preheat the oven to 350F. If you've left the dough overnight, take it out the dough a few minutes before you preheat the oven.
Portion out the dough using a 2oz cookie scoop and roll in a bowl of sugar. Space them at least 2-inches apart on a baking sheet lined with parchment paper or a silicone baking mat.
Bake for 10 - 12 minutes, or until the edges are a light golden brown. Leave the cookies on the baking sheet for 1 minute before transfering to a wire rack to cool completely.
Notes
To speed up the process of cooling the butter, you may place it in the refrigerator or freezer for 10 - 25 minutes.
Leaving the dough to chill overnight will result in the best texture and flavor development. I recommend leaving the dough to chill for up to three days.
If you plan on freezing the cookie dough, I recommend you allow the cookie dough to chill overnight or up to three days for flavor development, as this will not happen while the dough is frozen.