Heat oven to 170C fan-forced (190C conventional). Grease a 1.2-litre loaf tin with butter and line with baking paper. Reserve any leftover butter. In a bowl, lightly whisk together the eggs and sugar, then whisk in the vanilla extract and the oil. Gently whisk in the flour, salt and baking powder to form a batter.
Stir the apples through the batter and pour into the tin. Dot the reserved butter on top and scatter with the demerara sugar. Bake for 1 hour, or until a skewer inserted into the centre comes out clean. Cool for 10 minutes and remove from tin. Serve warm or at room temperature with whipped cream.
