1) Boil the balsamic vinegar and 3 tbsp of granulated sugar in a heavy small saucepan over medium-high heat until reduced to 120ml, stirring occasionally, about 20 minutes.
2) Toast the bread slices. Spread the butter over the toasts. Arrange the toasts on a platter. Drizzle the reduced balsamic syrup over the toasts then sprinkle with a little more granulated sugar, and serve.