Bake the cakes the day before you want to assemble them. Preheat the oven to 180C/350F/gas mark 4. Grease a 23cm/9in square tin and line with baking parchment. Line a muffin tin with fairy cake/small cupcake cases.
Sift together the flour, cornflour, bicarbonate of soda, cocoa powder and salt, set aside. Using an electric mixer, whisk together the butter and sugar until well combined. Whisk in the oil a little at a time, whisking well after each addition. Whisk in the egg yolks, vanilla extract and vinegar, beating until well combined.
With the mixture on a low speed, fold in one third of the flour mixture, followed by half the buttermilk, then another third of flour, the remaining buttermilk then the remaining flour.
Add red gel food colouring a little at a time until you reach the desired shade, bear in mind that the colour will darken a fair bit when the cakes are baked so you don't need to get it as dark as you want it to be.
In a clean bowl, vigorously whisk the egg whites with a balloon whisk until good and frothy but not yet beginning to stiffen then fold them into the cake batter.
Pour about three quarters of the mixture into the square tin then divide the remainder among the cupcake cases, I made five fairy cakes and three cupcakes.
Place the square tin on the lower middle oven shelf and the cupcake tray on the upper middle one. Bake the cupcakes for about 15 minutes, until a skewer inserted into the centre comes out clean; remove the cupcakes from the oven and transfer to a wire rack to cool. The large cake will take about 45 minutes to 1 hour to cook, cover loosely with foil towards the end of baking if the top starts to brown too much.
Place the tin on a wire rack and leave the cake to cool in the tin. Once cooled, store the cake and cupcakes in an airtight container.