This is one variation on a number of sweet potato soups I've been making for years. I love pairing spicy and smoky flavors with sweet potatoes, so curry blends are a natural. It's a semi-pureed soup, because I like the smoothness of a puree, but not by itself. The extra ingredients after the puree make the texture more interesting. —Bryan Lambert
To prep the sweet potatoes, dice half a pound of them into quarter-inch to half-inch cubes and reserve. Roughly chop the remaining sweet potatoes into one-inch chunks.
In a large pot, heat the vegetable oil over medium heat. Add the onions and cook for five minutes, until softened and lightly browned, about five minutes.
Add the garlic, ginger, and curry paste and stir thoroughly, cooking for 1-2 minutes. You can use your favorite blend, as long as it isn't overly spicy.
Add the large chunks of sweet potato, stirring them through the onions and spices, then add the chicken stock. Bring everything to a simmer, then let it simmer on low heat for 20 minutes, or until the sweet potatoes are soft.
Let the soup cool for 20 minutes, then, using an immersion blender, puree the soup until smooth. Adjust salt to taste - salt levels of both chicken broth and curry pastes vary.
Add the remaining sweet potatoes and corn to the puree, then bring it back up to a simmer. Simmer for another 15 minutes, until the dicdd sweet potatoes are tender.
Take the soup off the heat and let it cool for one or two minutes, then stir in the yogurt.