Heat the butter and oil in a large pot over medium-high heat.
Use a stick blender to puree the mixture until smooth. Alternately, transfer the mixture to a food processor or blender, puree and then transfer the mixture back into the pot.
Stir the chopped sage into the soup and cook the mixture for another 5 minutes.
Divide the soup among 4 bowls. Garnish with the reserved potato pieces and corn kernels. Sprinkle fresh sage or sage leaves over the top and serve.