Prep your tools and oven. Line two baking sheets with parchment paper. Preheat the oven to 350°F (175°C). Make sure your butter is soft but not greasy.

Cream the butter and sugars. In a large bowl, beat the butter, granulated sugar, and brown sugar together on medium speed for 2–3 minutes until light and fluffy. Proper creaming traps air and helps create that soft, chewy texture.

Add egg, yolk, and vanilla. Mix in the egg, then the extra yolk, followed by vanilla. Beat until smooth. Scrape the bowl so everything is well combined.

Whisk the dry ingredients. In a separate bowl, whisk flour, cornstarch, baking soda, baking powder, and salt. This ensures even distribution so your cookies rise consistently.

Combine wet and dry. Add the dry ingredients to the wet in two additions. Mix on low just until the flour disappears. Avoid overmixing to keep the cookies tender.

Chill briefly. Cover the bowl and chill the dough for 20–30 minutes. This quick rest firms the butter slightly and controls spread without making the dough hard to scoop.

Portion the dough. Scoop into 1 1/2-tablespoon balls (about a heaping tablespoon). Roll each ball smooth. If you like, roll in sugar or sanding sugar for a sparkly crust.

Arrange and bake. Place dough balls on prepared sheets, spaced 2 inches apart. Bake for 9–11 minutes, until the edges look set and just turning pale golden. The centers should look slightly underdone.

Tap and cool. For extra crinkles, gently tap the baking sheet on the counter once right out of the oven. Let cookies cool on the sheet for 5 minutes, then move to a rack to cool completely.

Optional finish. Once cool, drizzle with a simple vanilla glaze or frost lightly. Sprinkles are always welcome.