Preheat the oven to 400 degrees F and line a baking sheet with aluminum foil.
Place the plums cut side up on the baking sheet. Spread a thin layer of butter on top of each plum half and sprinkle with 1 teaspoon of brown sugar. Place the baking sheet in the oven and roast the plums for 15 minutes.
Remove the tray from the oven and allow the plums to cool briefly. Use a spatula to carefully move the plums onto serving plates without disturbing the gooey brown sugar tops.
Use a spoon to drizzle a small amount of crème fraîche over each plum. If using, top with a small amount of fresh ground black pepper. Serve immediately.
Whipped cream, sour cream or mascarpone can be used in place of crème fraîche.