Preheat oven to 180℃.
In a small bowl, whisk together plain flour and cinnamon, then set aside.
80 g plain flour (all-purpose flour), ¼ teaspoon ground cinnamon
To a medium bowl, add melted butter, brown sugar, golden syrup and vanilla, then mix to combine.
80 g salted butter, 70 g light brown sugar, 1 tablespoon golden syrup, 1 teaspoon vanilla extract
Mix in rolled oats.
60 g rolled oats (old-fashioned oats)
Then add the flour mixture and mix until combined. Set aside while you cut your apples.
Halve the apples from top to bottom, then scoop out the seeds and remove the stem, and hollow out some of the middle of the apples to create a bowl for the oat mixture (I use a melon baller for the scooping part).
2 large apples
Lastly, cut a small sliver (about 2cm in diameter) off the bottom of each apple half.
Place the apples into a baking dish, and divide the oat mixture evenly between the 4 apple halves.
Add ½ cup of water to the base of the baking dish.
Cover with a layer of baking paper, then a layer of aluminium foil.
Bake for 20 minutes, then remove the baking paper and foil (don't discard it yet, see my notes) and bake uncovered for a further 40 minutes until apples are soft and the crumble topping is browned.
Serve with a scoop of vanilla ice cream and a drizzle of caramel sauce if desired.
vanilla ice cream, caramel sauce