1. Preheat the oven to 170°C (150° fan) and grease and line six 20 cm (8-inch round) cake tins with greaseproof paper.*
2. Stir half the buttermilk into the oil and set aside. Beat the rest of the buttermilk into the egg whites with the salt and vanilla extract until smooth. Set aside.
3. Using a freestanding mixer fitted with a paddle attachment, mix the flour with the sugar, baking powder and bicarb. Add the butter and allow the mixer to "rub it in" on a low setting until it resembles fine breadcrumbs. Pour in the oil mixture with the paddle running, then increase the speed and beat for 2 minutes. Reduce the speed to low and slowly incorporate the egg white mixture.
4. Divide the cake batter between six bowls and colour each one a different colour, stirring until any streaks have disappeared. Scrape the batter into the prepared cake tins and level the tops.
5. Bake for 20 - 25 minutes, or until a skewer inserted in the centres comes out clean. Leave to cool for 10 minutes in the tins, then turn out onto wire racks and leave to cool completely.
6. Whisk the mascarpone, icing sugar, vanilla extract and salt with an electric whisk until smooth. Gradually whisk in the double cream, then whip until it just holds its shape (don't over-whip, as it will continue to stiffen as you colour and pipe it).
7. Weigh 200 g of the frosting into a bowl and colour it red. Put 100 g into another five bowls and colour them orange, yellow, green, blue and purple. Use the remaining white frosting to sandwich the cakes together and give them a thin crumb-coating round the outside. Chill in the fridge for 20 minutes to set the crumb coat.
8. Transfer each colour of icing to a piping bag fitted with a large star nozzle. Set the cake on a turntable. Holding the purple piping bag horizontally and at a 45° angle to the cake, pipe a line round the base of the cake. If you haven't reached the height of the purple cake, pipe a second layer. Repeat with the rest of the colours. Use the rest of the red frosting to cover the top: spin the turntable and pipe from the centre outwards.
9. Finish the cake with a scattering of silver cake sprinkles and some heart-shaped decorations.
Tips:
*If you don't have six separate cake tins, or you don't have enough space in your oven to bake them all at once, make and bake half the mixture at a time.
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