Preheat oven to 180 C / Gas 4. Grease and flour a 20x30cm baking tin. Sieve together the flour, cornflour and bicarbonate of soda; set aside.
In a large bowl, combine eggs, 200g caster sugar and pineapple with juice. Beat in the flour mixture. Pour into prepared tin and sprinkle over dark brown soft sugar.
Bake in preheated oven for 40 to 45 minutes or until a skewer inserted into the centre of the cake comes out clean. Remove from oven and prick top with a fork.
Meanwhile, make the topping. In a small saucepan combine the butter, evaporated milk, 200g caster sugar and 1 teaspoon vanilla. Cook over medium low heat, stirring occasionally until sugar is dissolved. Mixture will be thin. Pour over hot cake.