Preheat the oven to 180°C (fan-assisted).
Peel and dice 2 apples (keep the third apple for decoration, cut into thin slices).
In a bowl, whisk the eggs and sugar until pale.
Add the mascarpone, then the flour, baking powder, vanilla, and a pinch of salt. Mix until smooth.
Fold the diced apples into the batter.
Pour the batter into a greased and floured pan (or lined with baking paper).
Arrange the slices of the third apple attractively on top.
Bake for 35 to 40 minutes, until golden brown and a knife inserted comes out clean.
Let cool, then sprinkle with icing sugar before serving.