Preheat oven to 200°C. Brush a 2.5-litre ovenproof dish with the melted butter to lightly grease. Arrange apple pieces over the base of the dish.
Place caster sugar and water in a heavy-based saucepan over medium heat. Cook, stirring, for 2 minutes or until sugar dissolves. Increase heat to high and bring to the boil. Reduce heat to low and simmer, uncovered, for 10 minutes or until mixture is golden, occasionally brushing down side of pan with pastry brush dipped in water. Remove from heat, allow bubbles to subside. Pour toffee evenly over apples in dish.
Combine flour and sugar in a bowl. Whisk in eggs 1 at a time. Add the liqueur, zest, butter and vanilla sugar. Gradually pour in the milk, stirring continuously, until the mixture is smooth. Pour the mixture over the apples. Bake in preheated oven for 45 minutes or until a skewer inserted into the centre comes out clean. Remove from oven and set aside for 10 minutes before serving.
1.2kg golden delicious apples, peeled, cored, quartered
220g caster sugar
100g plain flour
2 tablespoons Cointreau liqueur
1 teaspoon orange zest
2 tablespoons melted butter
2 tablespoons vanilla sugar