Preheat oven to 180°C. Sift the flour, cornflour, baking powder and bicarbonate of soda into a bowl. Put half the butter in the bowl of an electric mixer fitted with the paddle attachment with the caster sugar for 5 minutes, until very light. Beat in the egg whites one at a time, then the oil, 2 Tbsp vanilla and the essences.
Add the butter mixture to the flours and mix gently, adding 1½ cups milk 2 Tbsp at a time, until the batter is smooth. Add the sprinkle and mix gently, until evenly distributed. Spoon into 1 lined 55cm x 40cm baking tray, or 6 x 20cm cake tins.** Bake for 25-28 minutes, until a skewer can be inserted and removed cleanly. Cool completely in the tin, then refrigerate for 30 minutes before proceeding.
Combine the remaining butter in the bowl of an electric mixer fitted with the paddle attachment with the icing sugar and remaining vanilla. Beat for 10 minutes, scraping down regularly, until very light and pale.
Use a 20cm cake hoop to cut three discs from the baking sheet (or cut by hand using a plate as a guide). Spread all three with a 4cm layer of frosting, then stack them up. Spread the remaining icing lightly on the sides, then scrape to reveal the sprinkle cake showing through. Decorate with Easter eggs.