Preheat oven to 200 degrees C. Grease a 23cm round cake tin with butter.
Mix 75g of the butter with the dark brown sugar and 4 tablespoons of the pineapple juice. Spread this mixture in the bottom of the prepared tin. Arrange the pineapple rings on top in a decorative pattern and set aside.
Stir together the flour, salt, caster sugar and baking powder.
Beat the egg whites in a bowl with an electric mixer until stiff peaks form. Set aside. In a separate bowl, beat two of the egg yolks until pale and creamy and then stir in the vanilla and the remaining pineapple juice and melted butter. Stir into the flour mixture until just combined. Gently fold in the egg whites. Pour batter over the top of the brown sugar and pineapple rings.
Bake for 30 minutes. Let cake cool for 10 minutes, then cover cake tin tightly with a serving dish and invert so that the cake is tipped out, pineapple side is up.