Bring medium pot of water to boil. Add 1 pound tomatoes; blanch 30 seconds. Transfer to bowl of cold water using slotted spoon. Drain tomatoes, peel, core and chop. Transfer to processor or blender.
Add yogurt, onion, sugar, 1 tablespoon basil, salt and cayenne; puree until smooth. Transfer to medium bowl. Cover and chill up to 8 hours. Garnish with basil and chopped tomato.