1.Preheat oven to 180°C/160°C fan-forced. Grease a 25cm springform cake pan; line the base and side with baking paper.
2.Beat butter, vanilla and brown sugar in a small bowl with an electric mixer until combined. Beat in the eggs, one at a time, then beat in sour cream. Stir in sifted flours and soda. Spoon mixture into the prepared pan. Smooth the top. Arrange the apple slices over the top.
3.Bake for 1 hour and 30 minutes or until a skewer inserted into the centre comes out clean. If the cake is browning too quickly, cover loosely with a layer of foil for the last 30 minutes. Cool in the pan.
4.Just before serving, place the caster sugar and water in a small saucepan. Stir over medium heat, without boiling, until the sugar is dissolved. Bring to the boil; boil, uncovered, until the toffee is beginning to turn golden brown, occasionally swirling the pan, but not stirring the mixture. Quickly remove from the heat and spoon the toffee over the apple.
Cake without toffee suitable to freeze. Not suitable to microwave.Note