2) Combine 200g of the caster sugar and the water in a small saucepan and cook over high heat until it turns a warm amber colour, about 182C on a sugar thermometer. Swirl the pan but don't stir. Pour evenly over the apple slices.
3) Meanwhile, cream the 90g butter and the remaining 150g caster sugar in the bowl of an electric mixer fitted with the paddle attachment, until light and fluffy. Lower the speed and beat in the eggs 1 at a time.
4) Add the sour cream, zest and vanilla and mix until combined. Sift together the flour, baking powder and salt and, with the mixer on low speed, add it to the butter mixture. Mix only until combined.
5) Pour the cake batter evenly over the apple slices and bake for 30 to 40 mins, until a cake tester comes out clean. Cool for 15 mins, then invert the cake onto a flat plate. If an apple slice sticks, ease it out and replace it in the design on top of the cake. Serve warm or at room temperature, dusted with icing sugar.