Preheat oven to 350. Line 24 muffin tins with paper liners. Set aside.
Whisk flour, cornstarch, baking powder and salt together in a medium bowl. Set aside. In a large bowl, cream together butter and sugar until light and fluffy. Add vanilla. Add eggs one at a time. Alternate adding dry (flour mixture) and wet (buttermilk) ingredients (dry-wet-dry-wet-dry) until just combined and no lumps remain. Do not overmix.
Fill prepared tins 2/3 full. Bake until a toothpick inserted in the center comes out clean, about 20-25 minutes. Cool completely.
To make the frosting, beat butter until light and fluffy. Beat in powdered sugar 1/2 cup at a time. Add vanilla then add buttermilk one tablespoon at a time until you reach your desired consistency. If frosting is too thin, add more powdered sugar; if it is too thick, add more milk.
Frost cooled cupcakes and top with a generous pinch of sprinkles. Serve immediately or store refrigerated in an airtight container for 3-5 days.
*I use buttermilk in my buttercream because I like the way the tanginess cuts the overwhelming sweetness. I always use reduced fat buttermilk and it comes out great. Feel free to use heavy cream if you prefer.