Preheat the oven to 300 degrees F. Butter an 8-inch square or 2-quart shallow baking dish.
Toast the bread slices on a baking sheet, turning occasionally, until lightly browned, 20 to 25 minutes.
Whisk together the eggs, half-and-half, pumpkin puree, brown sugar, pie spice, vanilla and salt in a large bowl. Add the toasted bread and let soak about 25 minutes, making sure that the bread stays submerged in the mixture.
Transfer the bread to the prepared dish and spoon the remaining mixture on top. Bake until firm and a toothpick inserted in the center comes out clean, 60 to 70 minutes.
Serve warm or at room temperature, dusted with confectioners' sugar and topped with whipped cream, if desired.