Combine squash and broth in a heavy 4-quart stockpot. Cook over medium heat unti squash is tender, about 15 minutes. Using a slotted spoon, transfer squash to a blender; puree until smooth. Stir broth back into pureed squash; set aside.
Melt butter in stockpot; stir in flour and curry powder. Cook over medium heat, stirring until smooth. Add squash mixture; increase heat to medium-high and stir until soup thickens slightly. Reduce heat to low; add remaining ingredients and heat through without boiling. Garnish, if desired.