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Method

1. To make the sponge, preheat the oven to 180C/160C Fan/Gas 4. Use a tiny knob of the butter to grease a 20x30cm/8x12in cake tin and then line with baking paper.

2. Put the butter, caster sugar, vanilla and lemon zest in a large bowl. Beat with an electric whisk until the butter is pale and fluffy. Beat in the eggs, one by one. Add the flour, baking powder and soured cream and use a spatula or metal spoon to mix to a smooth batter.

3. Scrape the cake mixture into the tin and bake for 22–25 minutes until a skewer poked into the centre of the cake comes out clean. Leave the cake to cool in the tin for 10 minutes, then transfer to a wire rack to cool completely.

4. To make the icing, place the butter and icing sugar in a large bowl. For a lemony icing, add the lemon juice and milk or for a plain icing, just use milk. Beat with an electric whisk until smooth, pale and fluffy, then spread smoothly over the top of the cake with a spatula.

5. To decorate, use the strawberry slices to make the central cross of a Union Jack on top of the cake. Add the raspberries to form the diagonal lines at the corners. Fill in the rest of the gaps with blueberries. The cake will keep in an airtight tin in a cool place for 3 days (do not place in the fridge). It is best decorated on the day of serving as the cut strawberries will bleed into the icing over time.