Soak the raisins in rum overnight. The next day, heat the oven to 180C (170C fan)/375F/gas 5. Grease and line four 20cm sandwich tins.
Put the sugar, oil and vanilla in a large bowl and beat with an electric whisk or a stand mixer with a beater attachment for two minutes until well combined. Mix the yoghurt and vinegar together in a separate bowl, then add this to the mixture and beat for a further two minutes. Add the dry ingredients and whisk until smooth, then gently fold in the rum-soaked raisins. Divide the mixture equally between the tins, and bake for 25 minutes, or until springy to the touch. Cool on a wire rack.
To make the buttercream, beat the soy butter with an electric mixer for a couple of minutes until smooth. Sieve the icing sugar in a separate bowl and add to the soy butter in two stages, beating well between each addition. Add the vanilla and a splash of soy milk to loosen if too thick. Divide between two bowls and set aside.
Once the cakes have cooled, use a sharp knife to trim the tops to level them off. On a board slightly bigger than your sponges, spread a small amount of buttercream and place the first layer of sponge on it. Spread a good dollop of buttercream on top then place another sponge on it. Repeat with the other sponges, but add the last one upside down. Spread a thin layer of buttercream with a palette knife over the whole cake to seal in the crumbs. Refrigerate for at least an hour.
Divide the second bowl of buttercream into two bowls. Colour one with the green food colouring, the other with the yellow, mixing in well. Apply the coloured buttercreams alternately around the cake, smoothing with a palette knife, to create a marbled effect. Any remaining buttercream can be used to pipe kisses around the top.
