Heat sugar and butter together in heavy-based pan, allowing sugar to dissolve and butter to melt. Add condensed milk, vanilla and salt; slowly bring to boil, stirring constantly. Cook for 10-15mins or until thick. Leave to cool a little.
Meanwhile, sit apples in large bowl of boiling water for 3-5mins to get rid of any wax. Line baking tray with buttered foil.
Dry and polish apples. Push chopstick/lollipop stick/wooden skewer into base of each apple. Dip apples into cooled caramel, dunking and twisting until well covered. Place on tray and allow to set.