For the baked apples, preheat the oven to 170C/150C Fan/Gas 3½. Grease a baking tray with a little butter.
Melt the butter. Brush the apples with the melted butter and roll in the caster sugar to coat all over. Place on the greased tray and bake in the oven for 25–30 minutes, until almost tender and still holding their shape. Set aside.
For the soufflé, preheat the oven to 190C/170C Fan/Gas 5.
Melt 10g/¼oz butter. Grease the baking dish with the melted butter, then sprinkle with 20g/¾oz sugar. Wipe the rim of the dish with a damp cloth, this will stop the soufflé sticking to the rim and enable it to rise evenly.
Pour the milk into a saucepan, add the vanilla and bring to the boil. Lower the heat and add 60g/2¼oz sugar, the semolina, sultanas and calvados (if using). Simmer for 3 minutes, or until thickened, whisking constantly to keep it smooth. Transfer to a large bowl, sprinkle with a little sugar to prevent a crust forming and let the mixture cool slightly, for 2-3 minutes.
In a clean bowl, whisk the egg whites with the lemon juice until soft peaks form and then slowly add the remaining 20g/¾oz caster sugar, whisking constantly. Continue to whisk until stiff peaks form when the whisk is removed.
Mix the egg yolks into the warm milk mixture, then briskly whisk in a third of the whisked egg whites. Carefully fold in the remaining egg whites using a spatula or large metal spoon. As you fold in the egg whites, delicately cut and lift the mixture to ensure there is minimum loss of volume and lightness.
Pour the mixture into the prepared baking dish and place the baked apples in it. Dab a knob of butter on top of each apple and sprinkle caster sugar evenly over the surface of the pudding. Bake for 25 minutes or until the soufflé is risen and golden-brown. Sprinkle with icing sugar and serve.