Grab a medium bowl and add your flour, cornstarch, baking soda, and salt. Whisk these together for about 30 seconds until everything is evenly distributed. This step prevents lumps and ensures the leavening agent spreads throughout the dough. I like to whisk in a circular motion, making sure to scrape the bottom and sides of the bowl. Set this mixture aside—you'll use it in just a moment.

Place your room-temperature butter and sugar into the bowl of your stand mixer fitted with the paddle attachment. This step is crucial: butter should be soft enough to indent with your finger but not greasy or melting. Beat on medium-high speed for 3 to 4 minutes until the mixture is pale, fluffy, and noticeably lighter in color. You're incorporating air into the butter, which helps the cookies rise and creates that tender crumb we love. Stop the mixer halfway through and scrape down the sides of the bowl with a spatula. If you're using a hand mixer, this takes about 5 minutes. If you're mixing by hand, plan for 8 to 10 minutes of steady beating—your arm will get a workout, but your cookies will thank you.

With the mixer still running on medium speed, crack your room-temperature egg directly into the butter mixture. Let it beat for about 30 seconds, then add your vanilla extract. Beat for another 30 to 45 seconds until the mixture looks smooth, creamy, and fully combined. Stop the mixer and scrape down the bowl once more. This ensures every bit of egg is incorporated, which means you won't see any streaks of egg white in your dough.

With the mixer on low speed, add your whisked dry ingredients to the butter mixture. Mix for about 1 minute until the flour is almost completely incorporated—you should see just a few streaks of flour remaining. Stop the mixer and scrape down the bowl. Don't overmix here; you want a tender cookie, and overmixing develops gluten, which makes them tough.

Now for the fun part. With the mixer on the lowest speed, add your rainbow sprinkles and mix for just 15 to 20 seconds until they're evenly distributed throughout the dough. Use your spatula to gently fold in any remaining flour you see at the bottom of the bowl. This is where a little patience pays off—gentle folding preserves the tender texture and ensures the sprinkles stay intact. Pro tip: Save a small handful of sprinkles for topping. These fresh sprinkles will stay brighter during baking and add extra visual appeal.

Using a 2-tablespoon cookie scoop (about 40g), portion out your dough onto a parchment-lined baking sheet. Roll each portion gently between your palms to create a smooth ball. Don't overwork the dough—these should be rustic, not perfect spheres. Top each ball with a small pinch of your reserved sprinkles, pressing them gently into the dough so they stick. Space the cookies about 2 inches apart on your baking sheet. They'll spread slightly during baking, so this spacing prevents them from baking into each other.

Place your baking sheet in the refrigerator for at least 1 hour. This chilling time is non-negotiable if you want the best results. Cold dough spreads more slowly, allowing the cookies to rise before they spread too much, which gives you that perfect puffy texture. You can chill for up to 24 hours—actually, chilled dough often bakes even better the next day as the flavors develop. Toward the end of your chilling time, preheat your oven to 350°F. Make sure your oven rack is positioned in the center for even heat distribution.

Place your chilled cookie sheet directly into your preheated 350°F oven. Bake for 10 to 12 minutes. You're looking for cookies that are puffed, with edges that look slightly set and firm, but centers that still jiggle very slightly when you gently shake the pan. The tops might look barely underdone—that's exactly right. Every oven is different, so I recommend checking at the 10-minute mark. If your oven runs hot, they might be done sooner. If it runs cool, they may need the full 12 minutes. The key is that the edges should be set while the centers remain tender.

Remove the baking sheet from the oven and let the cookies rest right on the sheet for 5 to 7 minutes. Don't move them yet. They're still baking gently from residual heat, and this time allows them to set just enough to handle without falling apart. After 5 minutes, you can transfer them to a wire cooling rack if you'd like, or leave them on the sheet until they're completely cool. This cooling time is what creates that signature texture: soft centers with slightly firmer edges. The longer they sit on the warm baking sheet, the more they set. The faster you move them to a cool surface, the softer they'll be throughout.