Place procini in small bowl; pour 1 cup boiling water over. Let stand until soft, about 45 minutes. Drain, reserving soaking liquid. Chop porcini; set aside.
Heat large nonstick skillet over medium heat. Add sausage, saute until brown, breaking up into small pieces with back of fork. Add porcini, shallots, and garlic. Reduce heat to medium-low; cover and cook until sausage is cooked through, stirring occasionally, about 5 minutes.
Transfer sausage mixture and any juices to medium bowl. Stir bread cubes, spinach, and rosemary into sausage mixture. Season with salt and pepper. Mix in 1/4 cup reserved soaking liquid. Cover and chill dressing.overnight.
Preheat oven to 325 degrees.
Bring dressing to room temperature. Whisk egg to blend in small bowl and blend into dressing. Spoon into greased casserole dish. Cover with foil and bake 25 minutes. Remove foil, dot top of dressing with butter pieces and bake, uncovered for additional 20 minutes. Serve.