Using a whisk or electric mixer, beat eggs, 3/4 cup sugar and vanilla in a large bowl to blend. Gradually mix in 1 1/2 cups heavy cream and 1 cup whole milk.
Place croissant pieces into prepared baking dish. Spoon on the Bittersweet Chocolate Sauce (in spoonfuls here and there). Pour egg mixture over croissants and chocolate. Let stand for 30 minutes.
Preheat oven to 350° F. Drizzle remaining cream over bread pudding and sprinkle with 1 tablespoon of sugar.
To make a water bath place baking dish in a large roasting pan. Fill with enough hot water to come halfway up the sides of the bread pudding baking dish. (If you choose not to use a water bath, cook 30 minutes or until the custard is golden and set in the center. Without the water bath the edges will be crispier.)
Bake until pudding is puffed and golden and custard is set in the center, about 50-60 minutes. Cool and serve warm.