Chop shallots, set aside. Clean and split leeks, split each leek into thirds and chop. Melt butter and heat oil over medium high heat in a large Dutch oven. Add shallots, leeks and salt. Stir periodically until covered with oil. Turn heat down to medium low, cover and sweat leeks for 20 minutes. Add chopped potatoes, chicken broth and vegetable broth. Bring to a boil and boil for 2 minutes. Drop heat to low and simmer for 40 minutes. When potatoes are done, remove from heat. With an immersion blender puree potatoes and leeks. Add yogurt, puree until smooth. Add white pepper, salt and pepper to taste. Serve.