Place a large skillet over medium heat. Add the butter and salt and cook until the butter is melted. Reduce the heat to medium-low, stirring frequently, for 5-8 minutes. The butter will be foamy, turn golden, and little brown bits will be in the bottom of the skillet.
Pour the butter into a large mixing bowl and stir to help cool it down. Let the butter rest for about 5 minutes.
Preheat your oven to 350 degrees F. Line two baking sheets with parchment paper or a silicone baking mat. Set aside.
Add the sugar and brown sugar to the cooled butter. Beat for 1 minute. The mixture will look grainy. Add the egg and vanilla and beat for 2 minutes until fluffy and lighter in color.
In a medium mixing bowl, stir together the flour, cornstarch, cream of tartar, and baking soda. Add the dry ingredients to the wet ingredients and mix on low just until a soft dough forms.
Use a 1.5-tablespoon cookie scoop to portion the cookie dough. Roll each scoop into a ball and place them on the prepared baking sheets, about 2 inches apart. Bake for 10-12 minutes, until the edges begin to get golden brown and the top of the cookies is an even color across.
Remove from the oven and let cool on the packing sheet for 5 minutes before transferring to a wire rack to cool completely.
Notes
Store any leftovers in an airtight container at room temperature for 3-4 days, refrigerate for up to 1 week, or freeze for 2-3 months.
Adding a couple of teaspoons to cookies can help them maintain a fluffy texture instead of flattening out too much and becoming hard discs. If you wish to omit the cornstarch, be sure to chill the dough for 30-60 minutes before baking so that they don’t flatten too much.