Heat the oven to 180C/160C fan/gas 4. Butter a 20 x 30cm cake tin and line with baking parchment. Put the butter and sugar in a large bowl and beat with an electric whisk for a few minutes until pale and fluffy. Add the eggs, flour, milk and vanilla and beat again until you have a smooth batter.

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.