In a medium bowl, whisk together the flour, baking powder, baking soda, and salt until well combined. Set aside.
2 3/4 cups all-purpose flour, 1 teaspoon baking powder, 1/2 teaspoon baking soda, 1/2 teaspoon fine sea salt
In a large mixing bowl, using a stand mixer with the paddle attachment or a hand mixer, beat the softened butter and granulated sugar on medium speed for 2–3 minutes, until light, fluffy, and slightly pale.
1 cup unsalted butter, 1 1/2 cups granulated sugar
Add the egg, egg yolk, vanilla extract, and almond extract (if using). Beat on medium speed just until smooth and well combined, about 30–45 seconds, scraping down the sides of the bowl as needed.
1 large egg, 1 large egg yolk, 2 teaspoons pure vanilla extract, 1/4 teaspoon almond extract
Pour in the milk or heavy cream and mix on low speed for 10–15 seconds, until incorporated. The batter will look slightly looser.
2 tablespoons whole milk or heavy cream
Add about half of the dry ingredient mixture and mix on low just until almost combined. Add the remaining dry ingredients and mix again on low until no streaks of flour remain. The dough should be thick and soft but not sticky.
2 3/4 cups all-purpose flour, 1 teaspoon baking powder, 1/2 teaspoon baking soda, 1/2 teaspoon fine sea salt
Using a spatula, gently fold in 1 cup of rainbow jimmies sprinkles, and optional lemon zest or white chocolate chips if using, until evenly distributed without overmixing.
1 cup rainbow jimmies sprinkles, 1/4 teaspoon fine lemon zest, 2 tablespoons white chocolate chips
Cover the bowl tightly with plastic wrap or a lid and chill the dough in the refrigerator for at least 30–45 minutes, or up to 24 hours. A short chill gives slightly more spread; a longer chill yields thicker, chewier cookies.
When the dough is nearly done chilling, preheat the oven to 350°F (175°C). Line 2 large baking sheets with parchment paper or silicone baking mats.
Use a 1.5-tablespoon cookie scoop or tablespoon to portion the dough into balls. Roll each ball between your hands to smooth, then roll in a shallow bowl of extra sprinkles, pressing gently so they adhere. Place on prepared baking sheets about 2 inches apart.
1 cup rainbow jimmies sprinkles
Bake one sheet at a time on the center rack for 10–12 minutes, until the edges are set and just barely turning light golden, the centers are still pale and slightly puffy, and the tops look softly crinkled. Do not overbake; they should look slightly underdone in the center.
Let the cookies cool on the baking sheet for 5–7 minutes to finish setting, then transfer carefully to a wire rack to cool completely. If desired, use a round cookie cutter to gently scoot the edges into perfect circles while the cookies are still hot.