Cake tatin aka apple upside-down cake

Ingredients

• 2 Granny Smith apples, peeled, quartered, core removed
• 1 cup castor sugar
• 100g butter (25g chopped, 75g melted)
• ¼ tsp salt 
• 1 cup self-raising flour
• 2 tsp ground cinnamon
• 2 eggs
• ½ cup milk
• 1 tsp vanilla extract
• vanilla ice-cream, to serve