Arrange a rack in the middle of the oven and preheat to 350°F. Butter a 12-inch cast-iron skillet or 9x13 inch-baking and set aside. Cube the bottom half of the croissants into 1-inch pieces while leaving the tops intact.
In a large bowl, whisk together the eggs, half and half, maple syrup, vanilla, cinnamon, and salt. Add the cubed croissants and gently toss in the egg mixture. Spread across the bottom of the cast-iron skillet and dollop with about ⅔ of the blackberry jam and crème fraîche. Shingle the intact top halves of the croissants on top and press down to absorb the egg mixture. Pour any remaining egg mixture on top and dollop with the remaining jam and crème fraîche.
Transfer the skillet to the oven and bake for 45-50 minutes or until the bread pudding is golden brown. If the top begins to brown too quickly loosely cover with foil. Serve with fresh berries and maple syrup.