Whisk together flour, baking powder, baking soda, cream of tartar, and salt in a medium bowl. Set aside.

In a large bowl, beat butter and sugar together on medium-high speed until creamy, about 1 minute. Add egg and vanilla extract and beat another minute until combined. Scrape sides and bottom of the bowl as needed.

Add dry ingredients to wet ingredients on low speed until just combined. Fold in ½ cup of sprinkles. The dough will be thick and sticky.

Scoop ~2 tablespoons of dough and roll into balls. Roll each ball in sugar and place on a plate or lined baking sheet.

Cover and chill the dough for at least 2 hours or up to 4 days.

Preheat oven to 350°F (177°C) and line baking sheets with parchment paper or silicone baking mats.

Place chilled dough balls 3 inches apart on the baking sheet and bake 11–12 minutes, until edges are lightly browned. The centers will still look soft.

Allow cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.

Store at room temperature for up to 1 week.