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1
Preheat the oven to180 degrees C/350 degrees F/Gas 4.
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2
Grease and line a 20cm square cake tin (pan) with baking parchment.
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3
Mix the red paste food colouring and the buttermilk together.
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4
(Use paste food colouring to get a deep scarlet - liquid colouring cannot achieve this shade without spoiling the taste.)
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5
Using an electric mixer with a large bowl, cream the butter and sugar until pale and fluffy.
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6
One by one, add the egg yolks and vanilla extract.
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7
Sift the flour, cornflour (cornstarch), cocoa powder (unsweetened cocoa) and baking powder into a bowl.
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8
Pulsing with the electric mixer, alternate between sifting in one-third of the dry ingredients and pouring in one-third of the buttermilk mix at a time until just fully incorporated.
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9
Whisk the egg whites and salt in a clean bowl until they form stiff peaks.
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10
Beat one-quarter of the egg white into the cake batter quickly to break some air then carefully fold in the remaining egg white one-quarter at a time using a metal spoon.
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11
In a separate small bowl, mix the vinegar and bicarbonate of soda (baking soda) until bubbles form then swiftly but gently fold into the mix.
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12
Pour evenly into the tin (pan) and pause for a minute to allow the bicarbonate of soda (baking soda) to work.
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13
Bake in the oven for 18-20 minutes or until a cocktail stick (toothpick) inserted comes out clean.
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14
Leave the cake in the tin (pan) for a few minutes before turning out onto a wire rack to cool completely.
