Step 1Cut squash in halves. Clean and roast at 450 degrees until very tender, about 1 hour. Scoop out flesh with spoon and set aside.
Step 2In large soup pot, saute onions in 1 tablespoon butter over medium heat until tender. Add leek and squash and cook 5 minutes. Add chicken stock, sugar, curry powder, cumin, salt and white pepper to taste.
Step 3Place soup mixture in blender, a batch at a time, blending until smooth, and return to pot. Blend last batch with 1 tablespoon butter and cream. Return to remaining soup, stirring, and heat through. Adjust seasoning and thickness.