Position a rack in the centre of oven and preheat to 180°C (160°C fan-forced). Grease a 23cm (base measurement) springform pan with the butter.
Cut each apple into 8 wedges, then into 5mm-thick slices. Place in a medium bowl. Add the Grand Marnier or orange-flavoured liqueur, if using, and toss to combine.
In a separate medium bowl, whisk the flour, caster sugar, baking powder, salt and 1/4 tsp of the cinnamon until well combined. In a large bowl, whisk the milk, oil, eggs, orange rind and vanilla until well combined. Add flour mixture to milk mixture and whisk until a smooth batter forms. (Don’t overmix.)
Transfer 1 cup (250ml) of the batter to a small bowl. Add the extra flour and whisk to combine.
Whisk the egg yolks into the remaining batter until just combined. Add the apple mixture and stir to combine. Spoon the apple batter into prepared pan. Spread evenly and press down on apple to submerge in batter. Pour reserved batter evenly over the top. Whisk the extra caster sugar and remaining cinnamon in a small bowl to combine. Sprinkle evenly over the batter.
Bake for 50 mins or until a skewer inserted into the centre of cake comes out clean and the top is golden brown. Transfer pan to a wire rack and set aside for 15 mins to cool slightly. Run a knife around the side of the cake and release side of the pan. Remove cake from pan and place on a wire rack lined with baking paper to cool.
Use an electric mixer to whisk the cream and icing sugar in a medium bowl until thick. Dust the cake with the caster sugar. Cut into wedges and serve with the whipped cream mixture.