Make the cake batter

• Beat the softened butter and caster sugar until pale and fluffy
• Beat in the eggs one at a time, then the vanilla extract
• Fold in the self-raising flour, then loosen with the milk

Bake the sponge

• Pour the batter into a lined baking dish and level the top
• Bake until golden and a skewer comes out clean

Make the icing

• Mix the sifted icing sugar with the water to a thick, spreadable icing

Decorate and serve

• Spread the icing over the cooled sponge
• Scatter the sprinkles on top and cut into squares