Preheat oven to 350 degrees and line a 12-cup muffin tin with cupcake liners. Set aside.
With a mixer, cream butter until light and fluffy, about 1-2 minutes. Add in granulated sugar, and cream the mixture until well combined.
Add in eggs one at a time, beating well after each addition.
In a separate bowl, sift together all purpose flour, corn starch, baking powder, baking soda and salt.
Alternately, add dry ingredients and 1/2 cup cereal milk to the butter mixture, beginning and ending with dry ingredients. Mix batter until just combined (over mixing cupcake batter can yield dry and crumbly cupcakes).
Mix in vanilla extract and sour cream.
Divide batter evenly into cupcake liners, filling each about 2/3 of the way full (you will have enough batter for 12-15 cupcakes, depending upon the size of your muffin tin).
Bake for about 18-20 minutes, or until a toothpick comes out clean.