Add the oil and butter to a soup pot over medium-high heat. Sauté the onion and celery for about 7-10 minutes or until it starts to brown lightly.
Stir in the garlic and cook for 30 seconds.
Add in the broth, sweet potatoes, and smoked paprika. Increase the heat to high and bring the soup to a boil. Cover the pot with the lid slightly open and reduce the heat and let it simmer rapidly for 25-30 minutes or until the sweet potatoes are soft.
Carefully blend the soup using an immersion blender or regular blender (you may want to let the soup cool a bit first).
Stir in the sour cream and heavy cream and season with salt & pepper as needed. The soup will likely be quite thick still, so thin it out with a bit more broth if needed. Taste and adjust... like add more cream or sour cream if needed.
Notes
I recommend cutting the sweet potatoes fairly small so they cook faster (no larger than 1" pieces or so).
I love the tanginess the sour cream adds to this soup, and you can't beat the creaminess of heavy cream. The combo makes this soup deliciously silky and offsets some of the sweetness of the potatoes. You can play with the quantities, omit one or both of the creams altogether, or swap for a cup or so of coconut milk if desired.