Place rack in upper third of oven; preheat oven to 400°F. Coat a baking sheet with cooking spray.
Make biscuits: Combine flour, cornstarch, sugar, baking powder and salt. Using your fingers, 2 knives or a pastry blender, cut in butter until mixture resembles coarse meal. Stir in milk with a fork just until mixture is evenly moistened.
Spoon heaping 1/4 cupfuls of dough onto prepared baking sheet to make six rounds. Sprinkle tops with cinnamon sugar. Bake until golden, 18 to 20 minutes. Let cool on a rack.
Make filling: Melt butter in a heavy skillet over medium heat. Add apples and cook, stirring, until tender, 8 to 10 minutes. Add brown sugar, cinnamon and salt; cook until sugar has melted, about 1 minute. Combine apple juice, rum and cornstarch in a small bowl. Add to skillet and cook, stirring, until thickened, 3 to 4 minutes. Allow sauce to cool and thicken, about 5 minutes.
To serve, cut biscuits in half horizontally. Place bottoms Fresh Apples Foster Shortcake on individual serving plates. Spoon apple mixture onto biscuit bottoms and replace biscuit tops. Serve warm with whipped cream, if desired.