Preheat the oven to 350 degrees F, and butter 4 cocottes (or a 6-cup baking dish).
Place the bread slices on a sheet pan in an even layer, allowing them to dry out overnight (or place them in a warm oven for about 20 minutes).
Spread a thin layer of salted butter on each of the bread slices.
Arrange the buttered bread in your baking dish(es), sprinkling with dried cranberries (if using).
Place the milk, cream, unsalted butter, granulated sugar, and salt in a medium pot and place over low heat until steaming.
Whisk the eggs and yolks together in a large bowl, then ladle in the hot milk mixture, a little at a time, to warm the eggs.
Stir in the vanilla.
Pour the custard over the buttered bread.
Sprinkle with Demerarra sugar, and place the baking dish(es) into a larger pan. Carefully pour hot water into the larger pan, to a depth of about 1 ½ inches.
Bake for 25 to 30 minutes (for individual bread & butter puddings)*, or until the edges of the bread have browned slightly but the custard is still a little wobbly towards the center.
Sprinkle with powdered sugar for garnish, and serve warm.