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Scrape the batter into the prepared tin, spread right to the corners and smooth the surface. Bake for 40-45 mins or until a skewer inserted into the centre comes out clean, checking at 5 min intervals if it isn't ready after 45 mins. Leave the sponge to cool on a wire rack.
Mix the icing sugar with enough water to make a thick icing. Spread the icing over the cooled cake, then top with lots of sprinkles – the surface should be almost completely covered. Leave for at least 1 hr to set. Cut into squares and serve with hot custard, if you like.