Melt butter in large saucepan over medium heat. Cook leeks 3 minutes. Add squash, curry powder and sugar; cook 2 minutes.

Add broth; cover and bring to a boil. Reduce heat and simmer 15 minutes or until squash is soft. Remove from heat.

Puree squash mixture with immersion blender or in food processor or blender until smooth. Serve with sour cream or Greek yogurt and chives, if desired.