Heat the oven to 175 ºC (350ºF). Grease and base line a 20cm round cake tin.
In a bowl, mix together the flour, 1/2 cup sugar, baking powder and salt. Mix well to combine.
Put the egg yolks and vanilla into a bowl and whisk until pale and thick. Whisk in the milk and oil, stirring until well combined. Stir in the dry ingredients.
Put the egg whites and cream of tartar together into another bowl and whisk to soft peaks. Gradually whisk in the remaining sugar. Fold a third of the egg white meringue into the mixture to loosen. Then fold in the remaining egg whites.
Pour the mixture to the prepared cake tin. Bake for 30 minutes, or until done. Remove from the oven and leave to cool. Remove from the cake tin. Cut into two layers horizontally.
To make the topping, put together all the ingredients in a saucepan. Bring the mixture to the boil and then turn down to the lowest possible heat, stirring continously, for 10-15 minutes until creamy and spreadable.
Use some icing to spread into one of the layers. Stack the layers together. Spread the remaining icing over the top and sides of the cake. Decorate and serve.
