Preheat the oven to 180ºC (fan 160ºC)/350ºF/gas mark 4. Grease a 22cm (8½in) sandwich tin with extra butter, and base-line with baking paper.
Step 2To make the sponge, cream the butter and sugar together in a large bowl using an electric hand whisk, until they are pale and fluffy. Add the eggs, one at a time, still whisking. Once all the eggs have been added, pour in the milk and whisk to combine. Sift the flour on to the mixture, and fold it and the grated citrus zest gently into the mixture using a large metal spoon.
Turn the mixture into the prepared sandwich tin, smooth the top, and bake in the preheated oven for 50 minutes or until a skewer inserted into the centre comes out clean. Leave to cool on a wire rack for 10 minutes while still in the tin, then carefully turn out of the tin, remove the paper and leave to cool completely.
To prepare the sugar syrup drizzle, mix the icing sugar and citrus juices together well.
When the cake is cool, make tiny holes all over the top of the cake with a skewer. Gently pour the syrup over the top and into the holes until it is absorbed and starts to form a nice white crust.
Finely grate lemon and lime zest with a Microplane over the top of your cake, and serve.