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Heat the oven to 180C/160C fan/ gas 4 and line a 23 x 33cm baking tin with baking parchment. Beat the butter and sugar together until light and fluffy using an electric whisk, about 4-5 mins. Add the eggs and flour, then beat again until smooth. Divide the batter between two bowls. Add the cocoa powder to one and the vanilla to the other, then mix each bowl well to combine.
Drop alternating spoonfuls of the chocolate and vanilla batters into the prepared tin until everything has been used up. Use a skewer or the tip of a cutlery knife to slightly swirl the batters together for a marble effect. Bake for 45-50 mins, or until a skewer inserted into the middle comes out clean. Leave to cool completely in the tin.
For the buttercream, beat the butter using an electric whisk until pale and soft. Sift in the sugar, a large spoonful at a time, and beat on a low speed to combine. Divide the buttercream between two bowls. Stir the vanilla into one and the cocoa into the other, and beat until light and fluffy. (Add 1-2 tbsp boiling water to loosen, if needed.)
Cut the cake into squares. Spoon the vanilla buttercream into one side of a piping bag fitted with a star nozzle and the chocolate into the other side. Pipe swirls over the squares to serve. Will keep in an airtight container for up to three days.