Dissolve bouillon cube in boiling water; set aside. Sauté onion, parsley, and celery leaves in butter in a heavy skillet until onion is tender. Add 1/3 cup bouillon, celery, marjoram, salt, and pepper, mixing well.
Bring to a boil. Reduce heat; cover and simmer 20 minutes.
Dissolve cornstarch in remaining % cup bouillon; add to celery mixture, stirring well. Bring to a boil; cook, stirring constantly, until thickened and bubbly.