Place sliced apples, lemon juice, butter, and dates in a saucepan over medium heat and cook until the apples soften, 10 minutes, stirring once or twice to prevent apples from burning, and so they cook evenly.
Mix together tapioca flour, spices, salt, water and vanilla. Pour over the apples, remove from the heat, and stir until the filling thickens.
Divide the apple filling between the four tarts, and spread evenly around the baked base. Top with reserved crumble mixture and sprinkle with extra sugar, if using. Return to oven and bake for 10 - 15 minutes or until the topping is golden.
Cool the Apple Crumble completely in tin, remove sides and serve.