Preheat the oven to 180°c/160°c Fan/Gas 4. Grease and line the base of two 20cm sandwich tins.
Add all the cake ingredients to a bowl and whisk until light and fluffy. Divide the mixture evenly between the tins and level the tops.
Bake for about 26 minutes. Leave to cool in the tins for 10 minutes, then run a knife around the edges to free the sponges. Turn the cakes out and leave to cool on wire racks, then remove the baking paper.
Add all the butter icing ingredients in a bowl and whiz together until everything is combined and smooth.
Spread half of the mixture on one cake, taking it evenly to the edges. Sit the other cake on top and swirl the remaining icing on top. Dust with icing sugar and cut into slices to serve.
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