Top a 9 x 13 baking pan with heavy-duty aluminum foil. Cut 15 "x" marks on the top where you'd like the cupcakes to be. Make sure they are not too close together. Insert the ice cream cones into the marks you've made.
Mix together the flour, cornstarch, baking powder, baking soda, and salt together in a medium bowl. Set aside.
Beat together the butter and sugar until light and fluffy - about 5 minutes on medium-high speed.
Beat in one of the eggs and mix for about a minute. Add in the second egg and the vanilla, then beat for another minute.
Stir together the buttermilk and sour cream in a small bowl.
Add in one-third of the flour into the butter mixture and stir until just combined.
Add in half the buttermilk mixture and stir until just combined.
Add in half of the remaining flour and stir until just combined.
Add in the remaining buttermilk mixture and stir until just combined,
Stir in the remaining flour mixture.
Evenly divide the batter among the ice cream cones. They should be about ⅔ full.*
Bake until the cupcakes leave only small crumbs on a toothpick when inserted in the middle. About 16 - 18 minutes. If the tops of the cupcakes brown too quickly, or they need additional time, lower the temperature to 325°F.
Allow the cupcakes to cool completely.
Make the buttercream while the cupcakes cool.
Beat the butter until smooth. Add in the powdered sugar one cup at a time, mixing well between each addition.
Add in the vanilla, then the salt and mix well.
Add in the heavy cream, starting with two tablespoons, then adding more as needed to get the desired consistency.
Pipe the cupcakes with the frosting and decorate!
Video
Notes
*You may have to remove each cone to give them a good tap so the batter can reach all the way to the bottom of the cone. Alternatively, you can try adding the batter to a piping bag to pip in the batter. If you do this, do not overfill the piping bag otherwise the batter will just ooze out uncontrollably.