Preheat oven to 350°F and grease a standard 9-inch bundt pan with nonstick spray.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, sugar, and vanilla together on medium speed until light and fluffy, about 2 minutes. Make sure to scrape down the sides of the bowl as needed.
1 cup unsalted butter, 1½ cups granulated sugar, 1 tablespoon pure vanilla extract
Add the eggs, one at a time, until fully incorporated.
4 large eggs
In a small bowl, whisk together the flour, cornstarch, and baking powder.
2¾ cups all-purpose flour, ¼ cup cornstarch, 2 teaspoons baking powder
On 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!
1 cup milk
Gently fold in the sprinkles.
⅓ cup red, white, and blue sprinkles
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 20 minutes. Then run a knife around the edge and turn it out onto a wire rack to cool completely.