Preheat the oven to 180°c / gas 4.
Line a 12" x 8" rectangular tin with baking paper.
In a large mixing bowl, add the flour, cornflour, margarine, caster sugar, vanilla extract, eggs and 125 ml of milk in a bowl. With an electric whisk, mix until smooth.
Pour the batter into a tin and bake for 34-40mins or until the skewer inserted in the centre comes out clean. Turn out onto a wire rack and leave to cool.
Cut the cake into bite size squares and place in the fridge for 30mins.