Boil the balsamic vinegar and 3 tablespoons of sugar in a heavy small saucepan over medium-high heat until reduced to 1/2 cup, stirring occasionally, about 20 minutes.
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 sugar, and serve.