Lightly grease the bottom and sides of a 23cm (9 in) springform cake tin and line fully with baking parchment.
Melt the 60g butter in a small pot on the hob, or in a small bowl in the microwave.
Slice the grapes lengthways into quarters (or, halves for smaller grapes), and leave between 2 sheets of kitchen paper to remove excess moisture.
Whisk the egg whites to soft peaks, starting at low speed and gradually increasing to high speed as the colour and texture change. (‘Soft peaks’ means that when you lift the beaters out, the peaks flop over almost immediately.)
Whisk the egg yolks, sugar, cooled melted butter, oil, milk and vanilla on medium speed until creamy, about 3-4 minutes (no need to clean the beaters from the whites).
Sift the flour, cornflour, baking powder and spices over the yolk mixture and fold in briskly with around 3/4 of the orange zest. Leave to rest whilst the oven heats.
Preheat oven to 180 C / 160 C Fan / Gas 4.
Add 1/3 of the egg whites, folding briskly to loosen the mixture; then add the rest, folding very carefully so you don't lose the volume, until no white streaks or lumps remain.
Pour the cake mixture into your prepared tin and sprinkle over approximately 3/4 of the sliced grapes.
Bake for 15 minutes, then sprinkle over the remaining orange zest, remaining grapes and the demerara sugar.
Bake another 40 minutes or so, turning the tin mid-way for even baking. When the top is a deep golden brown and the centre feels quite firm to a light finger touch, remove from the oven to a cooling rack. Leave the cake until fully cold (at least 1 hour), then turn out and dust with icing sugar before serving.
I researched recipes from all over the world, and combined elements of many recipes to create this cake. All recipes branded as 'authentic' use whole grapes; but my usual crowd of tasters quite liked the sliced grapes.