In a large stockpot, combine the butter, olive oil, onions and garlic. Saute over medium heat until the onions are tender but not brown, about 10 minutes or so. Sprinkle with the paprika, stir, and cook for a minute.
Add the vegetable broth, potatoes, celery, bay leaf, salt, sugar and pepper. Cook, partially covered, until the potatoes are tender, about 15 minutes. Stir in the corn and scallions and cook for 2 minutes.
Scoop out 2 cups of the chowder and set it aside. Puree the remainder (if you have an immersion blender, it comes in handy since you can puree directly in the pot of soup) and return it to the pot. Stir in the reserved chowder, milk, thyme and cayenne pepper. Cook for 5 minutes. Serve hot with a spoonful of sour cream on top.