How to Make It
Bring 2 quarts water to a boil in a large saucepan over medium-high. Stir in 2 tablespoons salt until dissolved. Add ear of corn; cook until tender, about 4 minutes. Remove corn, and set aside until cool enough to handle, about 10 minutes.
Return water to a boil over medium-high. Add potatoes; cook until tender, about 15 minutes. Drain; let cool 15 minutes.
Combine onion and vinegar in a large bowl; let stand 10 minutes. Stir in celery, mayonnaise, chopped basil, chopped dill, pepper, and remaining 21/2 teaspoons salt. Cut corn kernels from cob; discard cob. Add corn kernels and potatoes to onion mixture; stir mixture, lightly mashing together, until creamy. Cover and chill at least 1 hour or up to 3 days. Season to taste with salt and pepper; garnish with dill sprigs.