Preheat oven to 350°F. Line baking sheets with parchment paper or silicone baking mats.

Combine flour, baking soda, baking powder, cornstarch, and salt in a medium bowl; set aside.

Cream butter and brown sugar in a large bowl until light and fluffy.

Add egg and egg yolk, mix well.

Add vanilla extract and mix until combined.

Gradually add dry ingredients to the wet ingredients, mixing until dough comes together.

Create 1.5 tablespoon balls of dough, roll in coarse sugar, and place on prepared baking sheets.

Bake for 10 minutes or until edges are golden. Cool on wire racks.