Place the apple pieces in a saucepan with the 2 tbsp of caster sugar and 1 tbsp cold water. Heat on a medium heat for 5-6 minutes, stirring occasionally, until the apples starts to soften. Turn off the heat.
Place the plain flour in a bowl and add the butter. Pinch/rub the butter into the flour until combined and mixture looks like breadcrumbs, with some large lumps still left in it (it adds more texture and crunch if you have some larger lumps still in there). Stir in the cinnamon.
Take 6 small glass jars (mine were 135ml or 4+3/4 oz) and place on a baking tray.
Now take 6 pieces of kitchen foil - about the size of A4 paper, and fold/roll each one into a cylindrical shape, about the size of a white-board marker (approx 10-12cm long with 1+1/2 cm diameter).
Hold one of the foil cyclinders stood up in the middle of your glass jar, and spoon the apple mixture into the jar, around the foil - so that the apple mixture comes up about 5/6 of the way up the jar.
Spoon the crumble mixture on top, so that it's a little higher than the top of the jar. The foil should now stay in place. Repeat with the remaining jars.
Place the crumbles in the oven and cook for 20-25 minutes until the crumble is golden. The mixture will shrink down a little in the jar as the apples soften further.
Take the tray out of the oven and carefully remove the foil pieces using a gentle twisting motion.
If you're serving the crumbles hot, you can either heat the custard and pour in the holes before serving, or you can pour into the holes cold and place back in the oven for another few minutes (this will give you a skin on the custard).
If serving cold, wait for the crumbles to cool, then pour in cold custard. Place a lid on the jars and refrigerate until ready to serve.
Nutritional Information is per serving, based on 7 small jars.