Preheat the oven to 350°F (175°C). Line 2 large baking sheets with parchment paper or silicone baking mats to help the cookies bake evenly and prevent over-browning.
In a large mixing bowl, beat the softened unsalted butter and granulated sugar together on medium speed for 2–3 minutes, until light, creamy, and slightly fluffy. The mixture should look pale.
1 cup unsalted butter, 1 1/4 cups granulated sugar
Beat in the egg and egg yolk, one at a time, mixing well after each addition. Add the vanilla extract and mix again. Scrape down the sides and bottom of the bowl to ensure everything is well combined.
1 large egg, 1 large egg yolk, 1 1/2 teaspoons pure vanilla extract
In a separate bowl, whisk together the all-purpose flour, cornstarch, baking powder, baking soda, and salt until evenly combined.
2 1/2 cups all-purpose flour, 2 teaspoons cornstarch, 1 teaspoon baking powder, 1/2 teaspoon baking soda, 1/2 teaspoon fine sea salt
Add the dry ingredients to the butter mixture in 2–3 additions, mixing on low speed just until combined. The dough will be thick. If it looks crumbly and does not come together when pressed with your fingers, add 1 tablespoon of milk or cream and mix briefly, adding up to 1 more tablespoon if needed.
2 1/2 cups all-purpose flour, 2 teaspoons cornstarch, 1 teaspoon baking powder, 1/2 teaspoon baking soda, 1/2 teaspoon fine sea salt, 1-2 tablespoons milk or heavy cream
Using a spatula or your mixer on the lowest speed, gently fold in 3/4–1 cup sprinkles until just distributed. Avoid overmixing so the sprinkles don’t break and bleed color.
3/4-1 cup sprinkles
Scoop the dough into balls about 1 1/2 tablespoons each. Roll each between your palms to smooth, then gently roll in extra sprinkles and coarse sugar, if using, to coat the outside. Place on prepared baking sheets, spacing about 2 inches apart.
3/4-1 cup sprinkles, coarse sugar
Place the baking sheets with the shaped dough balls into the refrigerator and chill for 20–30 minutes. This helps the cookies stay thicker and chewier instead of spreading too much.
Bake one sheet at a time for 9–11 minutes, or until the edges look just set and very lightly golden while the centers still look soft and slightly puffy. The cookies will continue to cook on the hot pan after you remove them from the oven.
Let the cookies cool on the baking sheet for 5–10 minutes, then transfer to a wire rack to cool completely. Serve warm or at room temperature.