Preheat oven to 350°F. Line two baking sheets with parchment paper or silicone baking mat.
In a stand mixer or with a hand mixer, beat together butter and sugar until light and fluffy, about 2 minutes. Add egg and vanilla; beat until combined.
½ cup unsalted butter, softened, ¾ cup granulated sugar, 1 large egg, 2 teaspoons pure vanilla extract
In a separate bowl, mix together flour, cream of tartar, baking powder, baking soda, and salt. Add this mixture gradually to the wet ingredients with the mixer running on low. If you’re using a hand mixer, stir the flour in by hand. Mix until just combined, scraping down edges as needed. Stir in sprinkles by hand.
1 ½ cups all-purpose flour, 1 teaspoon cream of tartar, 1 teaspoon baking powder, ½ teaspoon baking soda, ¼ teaspoon salt, ¼ cup sprinkles or jimmies
Form balls using approximately 1 tablespoon of dough each. Place on cookie sheets (12 to 14 per sheet) and bake for 10 to 11 minutes or until set.
Remove from oven, cool for five minutes, and transfer to a wire rack to cool completely before storing in an airtight container.
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Notes
Variation: Use different color sprinkles (jimmies) to celebrate any occasion.
Storage: Cool cookies completely before storing in an airtight container. They'll keep at room temperature for 4 to 5 days; freeze for up to 3 months.
Make ahead: Prepare cookie dough as directed. Form dough into balls and freeze on a tray until firm, 1 to 2 hours. Place in freezer-safe storage container or bag. Bake as directed (don't thaw), adding a couple of minutes to the baking time.