Preheat the oven to 350F and grease a standard 9” bundt pan with non-stick cooking spray.
Using a stand mixer fitted with the paddle attachment, beat the butter, vanilla, and sugar together on medium speed until light and fluffy-- about 2 minutes. Make sure to scrape down the sides of the bowl as needed.
Add the eggs one at a time and fully incorporate each one before adding the next.
In a small bowl, whisk together the flour, cornstarch, and baking powder.
At a low speed, gradually alternate adding the dry ingredients and milk. First add in ⅓ of the flour mixture. Then add in half of the milk, followed by another ⅓ of the dry ingredients. Then add in the rest of the milk followed by the rest of the dry ingredients.
Mix just until no large lumps of flour remain. You don’t want to overmix!
Add in the sprinkles and fold them in gently.
Transfer the batter to the prepared bundt pan and bake until a toothpick inserted into the center comes out clean-- about 55-60 minutes.
Allow the bundt cake to cool for 5 minutes. Then run a knife around the edge and turn it out onto a wire rack to cool completely.
Store in an airtight container in the fridge for up to 3 days.