Heat the butter and oil in a large stockpot, add the leeks, and saute over medium-low heat for 5 minutes. Add the potatoes, zucchini, chicken stock, salt, and pepper; bring to a boil; then lower the heat and simmer for 30 minutes. Cool for a few minutes and then process through a food mill fitted with the medium disc. Add the cream and season to taste. Serve either cold or hot, garnished with chopped chives and/or zucchini.