In a large bowl, whisk together flour, baking powder, cream of tartar, baking soda, and salt. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment, beat butter until smooth. Scrape down the sides of the bowl. Add sugar and vanilla and mix on medium speed until combined. Add egg and mix until incorporated.
Slowly add the flour mixture in three additions, mixing on low speed. When the dough is almost thoroughly combined, add in ¼ cup sprinkles and continue to mix.
Using a 1 ½- Tbsp cookie scoop, portion out the dough. Place remaining ½ cup sprinkles in a bowl. Coat cookie balls in sprinkles and place on a parchment line tray. Cover sprinkle covered dough with plastic wrap and chill in the fridge for at least 2 hours (up to overnight). Dough must be properly chilled to prevent cookies from spreading too much in the oven.
Preheat oven to 350° F. Place chilled cookie dough on a parchment line baking sheet. Arrange cookies about 2-inches apart from each other. Bake for 10 minutes until cookies are set along the edges. Allow to cool in pan for 5 minutes before transferring to a wire rack to cool completely.
Fill cooled cookies with ganache, sandwich, and enjoy!