Whisk together the milk, cream, sugar, egg, yolk, and vanilla bean seeds until the sugar dissolves. Arrange the brioche slices in a single layer in a shallow baking dish. Pour the milk-egg mixture over the bread, submerging the bread. Cover with plastic wrap and refrigerate until the bread is completely soaked, almost too soft to handle, for at least 1 hour and up to overnight.
When you’re almost ready to serve, preheat the oven to 400°F. Generously butter a small rimmed baking sheet. Arrange the apples in a single layer. Dot the apples with 1 tablespoon of the butter. Roast until golden brown and tender, about 10 minutes.
Meanwhile, melt the remaining 2 tablespoons butter in a large ovenproof nonstick skillet over medium-low heat. When the butter foams, use both hands to very carefully transfer the soaked brioche slices to the pan. Cook until golden brown, about 5 minutes, then carefully flip. Transfer to the oven and bake alongside the apples until golden brown and puffed, about 5 minutes.
Serve the French toast with the apples and the crème anglaise and caramel, if desired. Dust with confectioners’ sugar.
