Preheat the oven to 180° C/Gas Mark 4 and grease and line a baking tray with butter and greaseproof paper.
Start by making the sponge; beat 225g of the butter and all the caster sugar together until light and fluffy. Add the eggs one at a time, beating each one into the mixture. Then fold in the flour, all but 1tsp of the lemon zest and ½tsp vanilla essence, keeping lots of air in the mixture.
Pour the mixture into the baking tray, spreading the mixture out as evenly as possible with a spatula. Bake for about 20 minutes (no longer) until golden brown and slightly risen.
Check with a sharp knife to see if it's ready if it comes out clean, the cake is cooked. Allow to cool completely when done.
For the icing, beat the rest of the butter (200g) until soft and pale. Add the icing sugar, ½tsp vanilla essence and 1tsp lemon zest and beat until smooth and creamy.
As soon as the cake is cool, smooth the icing onto the cake with a spatula and then decorate with your choice of decorations. Now serve and eat!
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