How to Make It
In a small saucepan, combine 3/4 cup of the sugar and 3/4 cup of water and bring to a boil over moderately high heat, stirring occasionally, until the sugar is dissolved, about 3 minutes. Remove the simple syrup from the heat; let cool slightly.
Preheat the oven to 350°. In an 8-inch-square baking dish, toss the plums with 1/2 cup of the simple syrup and the vanilla bean and seeds. Let stand for 20 minutes.
Bake the plums for 20 to 25 minutes, turning once, until softened. Let the plums cool slightly in the syrup, then discard the vanilla bean. Cut each plum half into 4 wedges.
In a small bowl, whisk the remaining 2 tablespoons of sugar with the cinnamon.
In a shallow glass pie plate, whisk the remaining 1/2 cup of simple syrup with the heavy cream and eggs. Add the bread slices 1 at a time and soak until evenly moistened but not soggy, 2 to 3 minutes per side; transfer the slices to a plate while you soak the others.
In a large skillet, melt the butter over moderately low heat. Sprinkle half of the cinnamon sugar on one side of each
bread slice and cook sugared side down until the bread is browned, about 5 minutes. Sprinkle with the remaining cinnamon sugar, then flip and cook until browned on the other side, about 5 minutes longer. Transfer the French toast to plates and dust with confectioners’ sugar. Serve with the plums and their syrup and whipped crème fraîche.