In a large saucepan, toss apple, ¾ cup (165g) sugar, 2½ tbs lemon juice and vanilla to coat. Over medium-high heat, cook apple mixture, stirring often, for 5 mins or until apples release their juices and mixture begins to simmer. Reduce heat to medium-low and simmer gently, stirring occasionally, for 20 mins or until apples soften and begin to fall apart.
Puree mixture in a food processor. Return mixture to pan and set aside.
In a small saucepan over medium-high heat, heat remaining ½ cup (110g) sugar, without stirring, until sugar has dissolved and caramel is golden brown, swirling saucepan as needed to cook sugar evenly. Stir in salt and immediately stir caramel into apple mixture.
Bring apple butter to a simmer over low heat. Simmer gently, stirring constantly to prevent scorching, for 2 mins or until combined and any lumps of caramel have dissolved. Stir in remaining ½ tbs lemon juice. Set aside to cool slightly.In a small bowl, stir butter and spices until well blended. Spread butter onto cut sides of hot cross buns. Heat a large frying pan over medium heat. Cook half the buns, cut-side down, for 15-30 secs or until golden and toasted. Wipe frying pan clean. Repeat with remaining buns. Spread apple butter over toasted buns and serve warm.