Creamy Cannellini Soup

Turn on a multi-functional pressure cooker (such as Instant Pot®) and select Saute function. Once hot, add butter, followed by sausage and fennel. Stir and break up sausage. Cook, stirring, until sausage is browned, about 5 minutes. Pour in 1 cup chicken broth slowly, scraping up any browned bits from the bottom of the pot. Pour in 2 cans of drained cannellini beans. Cancel Saute mode. Close and lock the lid. Select high pressure according to manufacturer's instructions; set timer for 5 minutes. Allow 10 to 15 minutes for pressure to build. Meanwhile, combine remaining 2 cups chicken broth and 2 cans of beans in a blender; blend on high until smooth. Release pressure using the natural-release method according to manufacturer's instructions for 5 minutes. Release remaining pressure using the quick-release method according to manufacturer's instructions, about 5 minutes. Unlock and remove the lid. Add pureed beans, thyme, and truffle salt to soup; mix to combine. Ladle soup into bowls and serve with a drizzle of olive oil.