Melt butter in a sauce pan and saute shallot and thyme.
Cook until shallots are softened but not browned.
Add water to pan and increase heat to high, and bring to simmer.
Remove pot from heat and add sweet potatoes and reserved peels. Let stand 20 minutes.
Add sugar, vinegar and 1/12 teaspoons of salt and 1/4 teaspoon of pepper.
Bring to simmer over high heat.
Reduce heat to medium low and cook until potatoes are very soft, about 10 minutes.
Discard thyme sprigs.
If you have an immersion blender, then blend the soup until smooth. It will be very creamy. Best to let it cool to a safer temperature before blending.
If you do not have an immersion blender, then working in batches, process soup in a blender until smooth, 45 - 60 seconds. Return to a clean pan and bring to simmer.
Adjust consistency and seasoning if needed.