Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.
Cream the softened butter, brown sugar, and 1/4 cup granulated sugar together in a large bowl on medium speed for 2-3 minutes until fluffy.
Add the egg, egg yolk, and vanilla extract. Mix until smooth.
Whisk the flour, cornstarch, baking soda, and salt together in a separate bowl. Gradually add this to the wet ingredients, mixing on low until just combined.
Roll the dough into 1.5-tablespoon balls. Roll each ball generously in the extra granulated sugar.
Bake for 8-10 minutes. The edges should be set but the centers will look slightly soft. Let them cool on the pan for 5 minutes before moving to a wire rack.
