Cupcakes: Preheat oven to 160 degrees C. Line a cupcake tray with patty cups.
Mix the cornflour, plain flour, baking powder, sugar and salt in large bowl with a spatula. Stir in butter cubes until they are well coated with the flour.
Whisk eggs, milk and vanilla essence together in a tall container such as a large measuring jug. Pour one-third of the egg-milk mixture into the flour mixture and stir with the spatula. Add the remaining egg-milk mixture in thirds and stir with the spatula until well combined. The batter will be very lumpy and the butter might remain together.
Fill the patty cases 2/3 full with the batter and bake in the preheated oven for about 17-20 minutes. After 10 minutes turn around the cupcake tray so that the cakes bake evenly. Use a skewer to test if the cupcakes are done in the middle - when the skewer comes out clean they are ready.
Leave the cupcakes for 10 minutes in the tray and then cool completely on a wire cake rack.
Buttercream: Beat the butter for 2-3 minutes until fluffy. Mix in the milk and vanilla essence alternately with the icing sugar and then mix at low speed into the butter until the cream is light and fluffy. If the buttercream is too thin add more icing sugar a teaspoon at a time.
Spread the buttercream over the cooled cupcakes with a knife. If desired, decorate with grated chocolate or hundreds and thousands.