In a sauce pan, heat and mix the sugar, water, and butter to create a light brown caramel sauce. Remove from heat and set aside.
Heat the grill and core the large plums, leave the small plums whole. Put in the pan or roast on the grill for 5 minutes, turning constantly. Roast white bread on the grill, in the pan or toaster. Place toast on 4 plates, top with grilled plums and yoghurt. Pour the caramel sauce over it with a spoon and dust with cinnamon.