Preheat your oven to 350°F (175°C) and line two cookie sheets with parchment paper. This temperature ensures the cookies spread just right without burning.
Pour the entire cake mix into a large bowl, making sure to break up any lumps with your fingers or a whisk.
Add the oil, eggs, and vanilla extract to the bowl and mix until completely combined. The dough will be quite thick and somewhat sticky – this is exactly what you want!
Fold in the additional 1/4 cup of sprinkles gently with a spatula until evenly distributed. Avoid overmixing which can cause sprinkle colors to bleed.
Scoop the dough using a tablespoon or cookie scoop, forming balls about 1.5 inches in diameter. If the dough is too sticky, refrigerate it for 15-20 minutes.
Roll the dough balls in extra sprinkles if desired, then place on the prepared cookie sheets, spacing about 2 inches apart.
Bake for 9-11 minutes until the edges are just set but the centers still look slightly underdone. They’ll firm up as they cool, resulting in the perfect chewy texture.
Let the cookies cool on the baking sheets for 5 minutes before transferring to a wire rack to cool completely.