2) Cream the butter and granulated sugar in the bowl of an electric mixer fitted with the paddle attachment on high speed, until light and fluffy. On medium speed, add the eggs, 2 at a time, then add the sour cream and vanilla. Scrape down the sides and stir until smooth.
3) Sift together the plain flour, cornflour, salt, and bicarbonate of soda in a bowl. With the mixer on low speed, add the plain flour mixture to the butter mixture until just combined. Fill the cupcake liners to the top with batter. Bake in the center of the oven for 20 to 30 minutes, until a toothpick comes out clean. Cool to room temperature.
4) For the icing, mix the butter, cream cheese, icing sugar, and vanilla in the bowl of an electric mixer fitted with the paddle attachment, on low speed, mixing just until smooth. Spread the icing generously on top of each cupcake.