Heat oven to 425 degrees. Slice plums in half, and remove pits. Toss plum halves with 2 tablespoons brown sugar; place in an ovenproof skillet.
Transfer skillet to oven, and roast plums until they are soft and juice is bubbling, 15 to 20 minutes. Remove from oven, and let cool slightly.
Just before serving, place creme fraiche in a small, clean mixing bowl, and whip with a whisk until it holds a soft peak like whipped cream. Spoon plums into serving bowls. Top plums with a dollop of whipped creme fraiche. Sprinkle remaining tablespoon brown sugar over creme fraiche, and serve immediately.