2) Combine 220g of the caster sugar and 80ml of 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 plums.
3) Meanwhile, cream the 85g of butter and the remaining 170g of 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 one at a time. Add the sour cream, zest and vanilla and mix until combined.
4) 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. Pour the cake batter evenly over the plums.
5) Bake for 30 to 40 minutes, until a cake tester comes out clean. Cool for 15 minutes, then invert the cake onto a flat plate. If a plum 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.