In a large stockpot, combine the butter, olive oil, onions, and garlic. Sauté over medium heat until the onions are tender but not brown. Sprinkle on the paprika, toss, and cook 1 minute more.
Add the stock, 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 2 minutes more. Remove the bay leaf.
(This step is optional.) Scoop out 2 cups of the choder and set aside. Puree the remainder and return all the chowder to the pot.
Stir in the milk, thyme, and cayenne pepper. Cook 5 minutes more. Serve in bowls with a spoonful of sour cream on top.