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

In a large bowl using a hand mixer or a stand mixer fitted with a paddle attachment, beat the butter and granulated sugar together on medium-high speed until creamed, about 1 minute. Add the egg and vanilla extract and beat on high speed until combined, about 1 minute. Scrape down the sides and up the bottom of the bowl and beat again as needed to combine.

Add the dry ingredients to the wet ingredients and mix on low until combined. Beat in 1/2 cup of sprinkles. Dough will be thick and sticky. Scoop large sections of dough (about 2 Tablespoons of dough each) and roll into balls. If desired, lightly dip the tops of the cookie dough balls in more sprinkles. Place dough balls onto a large plate or lined baking sheet.

Cover and chill the cookie dough balls in the refrigerator for at least 2 hours (and up to 4 days).

Preheat oven to 350°F (177°C). Line baking sheets with parchment paper or silicone baking mats. Set aside.

Arrange chilled cookie dough balls 3 inches apart on the baking sheets. Bake for 11-12 minutes or until lightly browned on the sides. The centers will look very soft.

Remove from the oven and allow cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.

Cookies stay fresh covered at room temperature for up to 1 week.