Preheat the oven to 350° F. Line a baking sheet with parchment paper. Grease two muffin pans or line with paper baking cups, then set pans on baking sheet.
In a mixer bowl or food processor, cream the butter with the sugar until well blended. Blend in the eggs, vanilla, milk and sour cream. Fold in the flour, corn starch, baking powder, baking soda and salt and blend until smooth, about 3-4 minutes. Spoon into prepared pans. Bake at 350° F. for 25-30 minutes. Cool in pan 15 minutes before unmolding cupcakes to cool completely.
Top with buttercream frosting. For special occasions, add swirls of color to your buttercream frosting using this technique: https://clabbergirl.wpengine.com/butter-pecan-cookies-with-buttercream-swirls/.