Preheat the oven to 375 degrees F.
Line 2 large baking sheets with parchment paper.
In a large bowl with electric mixer, beat the butter until no longer in chunks.
Add the sugar and beat until combined, about 1 minute.
Add the eggs and egg yolk, one at a time, until combined.
Add the baking powder, baking soda, and salt, and stir to combine.
Add the cake mix and the flour, beating until combined.
Using a ½ cup measuring cup, form the dough into ½ cup cookie balls. Note: this recipe makes 8 cookies. If you turn out with more cookie dough balls, they aren't big enough! You can still bake them, but they just won't be as large.
Place the sprinkles in a large bowl and roll each dough ball in the sprinkles.
Place 3 of the balls on the prepared cookie sheet.
Bake in the preheated oven for 10-12 minutes. The edges will be slightly golden. The center may not look completely done, but it shouldn't be shiny on top. If the dough is still shiny, put them cookies in for another 1-2 minutes and check regularly. Cool completely (this is important!). Repeat with the remaining cookie dough.
Store in an airtight container.
Enjoy!