Remove corn kernels from cobs and place in a bowl. Scrape 'milk' from cobs into bowl with the kernels. Reserve 1 cob.
Melt butter in a large saucepan over medium-low heat. Add onion, scallions, and garlic; cook until soft but not brown, about 3 minutes. Add corn kernels, corn milk, potatoes, and jalapeno. Cook, stirring frequently, for 10 minutes. Add milk, broth, and reserved cob. Simmer until potatoes are completely soft and soup is slightly thick, 15 to 20 minutes. Remove and discard cob.
Season chowder with cilantro and pepper and serve.
