For the pudding: Preheat oven to 400°. Butter a medium baking dish with 1 tbsp. of the butter. Arrange bread in a single layer in the prepared dish and set aside at room temperature to dry out slightly, about 2 hours. Meanwhile, prick sweet potatoes in 4 or 5 places with the tines of a fork and bake on a baking sheet until soft, about 1 hour. Set aside until cool enough to handle, then halve lengthwise and scoop meat out of skins. If meat holds together, break it into large pieces. Tuck sweet potato pieces between the pieces of bread, mashing them down slightly with a fork.
Beat together milk, eggs, sugar, vanilla, and cinnamon in a large bowl. Pour over bread and sweet potatoes and set aside until bread soaks up milk mixture, 2–3 hours. Preheat oven to 375°. Cut the remaining 7 tbsp. butter into small pieces and scatter over bread pudding, then bake until custard is set, 35–40 minutes. Set aside to cool for at least 30 minutes before serving warm or at room temperature.
For the praline sauce: Melt butter in a medium, heavy-bottomed saucepan over medium heat. Add sugar and stir with a wooden spoon until sugar melts and mixture begins to boil, about 5 minutes. Stir in pecans and bourbon. Spoon warm sauce over bread pudding.