In a saucepan over medium heat, melt the butter, stirring constantly. Cook until it foams and brown bits form, 5-7 minutes.
¾ cup unsalted butter
Remove from heat and let cool 10-15 minutes. Skim off excess foam.
In a separate bowl, whisk together flour, baking soda, cream of tartar, cornstarch, and salt.
2 cups all-purpose flour, 1/2 teaspoon baking soda, 1 teaspoon cream of tartar, 1/2 teaspoon salt, 2 teaspoon cornstarch
Whisk the browned butter with granulated sugar and brown sugar until combined. Add egg, egg yolk, and vanilla. Mix until smooth.
1/3 cup light brown sugar, 1 large egg, 1 large egg yolk, 2 teaspoon vanilla extract, 1 cup granulated sugar
Stir into the wet mixture into the dry mixture.
Mix just until the dough comes together and everything is combined well.
Scoop dough with a cookie scoop (about 1.5 tbsp each), roll into balls, and coat in granulated sugar or cinnamon sugar. Chill sugar cookie dough balls for at least 1 hour.
1 cup granulated sugar, Cinnamon sugar for rolling or a mixture of cinnamon and granulated sugar
Step 8. Preheat oven to 350°F and line baking sheets with parchment paper. Place dough balls on prepared sheets with a little space between them.
Step 9. Bake 12-14 minutes, until edges are set and centers look slightly underdone.
Step 10. Let cookies cool on the pan for 5 minutes, then transfer to a rack to cool completely.