Preheat oven to 300°.
Generously butter a 9x13 baking dish.
Prepare a water bath by filling a larger baking dish less than halfway with water and placing it in the oven to heat.
Peel and cube sweet potato and boil until fork-tender. When mostly cooled, toss in 2 Tbsp. brown sugar and ½ tsp. cinnamon.
Begin scalded milk. Set aside when done to cool slightly.
Fill the buttered dish with bread cubes, and then scatter the sweet potato cubes and pineapple bits.
In a separate bowl, beat together eggs, white sugar, and vanilla.
Gradually add in scalded milk, whisking continuously. Be careful not to add hot milk too quickly, as it will cook the eggs. Pour over bread. Using the back of a large spoon, be sure all bread is gently pushed into the wet mixture.
Combine ¾ cup brown sugar, 1 tsp. cinnamon, nutmeg, cardamom, and pecans, then sprinkle over the top.
Optional marshmallows may be added at the last 10 minutes, or added after baking and toasted lightly using a kitchen torch.
Place baking dish in water bath and bake for 50-60 minutes or until a knife inserted in the center comes out clean.
