Put sliced rhubarb into a bowl and sprinkle with sugar. Let macerate for 1 hour. Butter the bread and arrange 4 slices, buttered side down, in a buttered dish. Sprinkle with half the rhubarb, arrange another layer of bread, buttered side down, add rest of rhubarb and cover with last 4 bread slices. In a mixing bowl, whisk together 2 cups cream, 1 cup milk, 4 eggs, vanilla and 3/4 cups sugar. Pour mixture through a fine sieve over the bread. Sprinkle sugar over the top and let mixture stand, loosely covered, for at least 1 hour or refrigerate overnight. Bake in a bain-marie in a pre-heated 350 degree oven for 1 hour or until the top is crisp and golden. Serve warm with softly whipped cream.