1. In a medium bowl, whisk together flour, cornstarch, baking soda, and salt. Set aside.

2. In a large bowl, cream the softened butter, granulated sugar, and brown sugar together on medium-high speed for 2–3 minutes until light and fluffy.

3. Scrape down the sides. Add the egg, egg yolk, and vanilla extract. Beat until well combined.

4. Reduce mixer speed to low. Slowly add dry ingredients to wet ingredients, mixing just until combined. Do not overmix.

5. Cover dough tightly with plastic wrap and refrigerate for at least 2 hours or up to 2 days.

6. Preheat oven to 350°F (177°C). Line baking sheets with parchment paper.

7. Scoop chilled dough into 1.5-tablespoon balls. Roll in sugar or sprinkles if desired. Place 2 inches apart on prepared baking sheets.

8. Bake for 10–12 minutes until edges are lightly golden. Centers should look soft and slightly underbaked.

9. Cool on baking sheet for 5 minutes, then transfer to a wire rack to cool completely.

10. Store cooled cookies in an airtight container for up to 5 days, or freeze for longer storage.