Preheat the oven to 350 F. Line a baking sheet with parchment paper. Generously spray a two cupcake tins with non-stick cooking spray and place on the 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, cornstarch, baking powder, baking soda and salt and blend until smooth, about 3-4 minutes. Spoon into prepared pan. Bake until cupcakes test done when gently pressed with fingertips, 25-30 minutes.
Cool in pan 15 minutes before unmolding cupcakes to cool completely. For the frosting, heat the cream until it starts to bubble. Add in the chocolate, turn off the heat, and stir to melt chocolate evenly into cream. Chill 2-4 hours until firm. Place in a mixer bowl and whip on low speed, adding in butter to fluff up the ganache. Spread on cupcakes.