Step 1

Place 1 stick unsalted butter in the bowl of a stand mixer (or large bowl if using an electric hand mixer.) Place 2 large eggs in a small bowl. Let sit at room temperature until the butter is softened.

Step 2

Place 2 2/3 cups all-purpose flour, 1 1/2 teaspoons baking powder, and 3/4 teaspoon kosher salt in a medium bowl and whisk to combine.

Step 3

Add 1 cup of the granulated sugar to the bowl of softened butter. Beat with the paddle attachment on medium-high speed until light and fluffy, scraping down the sides and bottom of the bowl as needed, 3 to 4 minutes. With the mixer running, beat in the eggs one at a time, waiting until the first one is fully incorporated before beating in the second. Continue beating until light, fluffy, and lightened in color, 2 to 3 minutes. Add 2 teaspoons vanilla extract and beat just until combined, about 1 minute.

Step 4

Turn the mixer off. Add the flour mixture and beat on low speed until a soft dough forms and no pockets of dry flour remain, 1 to 2 minutes. Add 1/2 cup rainbow sprinkles and stir by hand with a rubber spatula to evenly distribute throughout the dough. Cover the bowl tightly in plastic wrap and refrigerate for at least 2 hours or up to overnight.

Step 5

Arrange 2 racks to divide the oven into thirds and heat the oven to 350°F. Line 2 baking sheets with parchment paper.

Step 6

Place the remaining 1/4 cup granulated sugar in a medium, shallow bowl and 1/4 cup powdered sugar in a second medium, shallow bowl. Divide the dough into 16 portions (about 2 heaping tablespoons each), then roll each portion into a smooth ball. Toss the dough balls one at a time in the granulated sugar to completely coat, then toss to completely coat in powdered sugar. Place 8 dough balls on each baking sheet, spacing them at least 2 inches apart.

Step 7

Bake for 8 minutes. Rotate the baking sheets from top to bottom and back to front. Bake until the cookies crack and no longer look wet, 5 to 8 minutes more. (The cookies will barely brown.) Let the cookies cool completely on the baking sheets.