Preheat oven to 400 degrees F and spray a 12-cup muffin pan with nonstick cooking spray.
In a medium bowl, whisk together the flour, cornstarch, baking powder, baking soda, and salt. Set aside.
In a separate bowl, beat together the granulated sugar, brown sugar, and oil until smooth and well combined.
Add the eggs one at a time, beating between each addition.
Beat in the milk and vanilla.
Slowly add the dry ingredients and mix JUST until smooth. DO NOT OVERMIX!
Fold in the mini chocolate chips and sprinkles until well mixed.
Fill each cup of the muffin pan 2/3 of the way full. Sprinkle additional chocolate chips and sprinkles on top of each muffin, if desired.
Bake in the preheated oven for 18 to 20 minutes, until a toothpick inserted in the center comes out clean. The muffins will be a deep golden color on the outside.
Allow to cool in the pan until safe to touch, then transfer the muffins to a wire cooling rack to cool completely before serving.