Preheat the oven to 190ºC/375ºF/Gas Mark 5. Grease and line a nine-inch round cake tin. Mix soft butter and demerara sugar and spread it in at the bottom of the cake tin. Arrange a layer of pineapple slices over the butter sugar mixture.
For the cake batter, sift together the flour and baking powder. Beat the egg yolks in a small bowl; mix in the butter and vanilla essence. Whisk the egg whites until stiff but not dry. Fold in the sifted sugar, one tablespoon at a time. Fold the egg yolk mixture into the egg whites, then fold in the sifted flour, one fourth cup at a time. Pour the batter over the pineapple slices and bake in the tin and bake for about half an hour. Allow to cool a little before removing from the tin.
Turn the cake upside- down, onto a serving dish with the glazed pineapple slices on top.