Meanwhile, place corn cobs in large pot and add 8 cups water. Bring to simmer and cook covered until water is corn-flavored, about 45 minutes. Remove cobs and discard.
Reserve 1/2 cup roasted corn for relish. Working in batches, place one third of remaining corn in blender. Add 1 1/3 cups corn liquid; cover and carefully blend until smooth. Place in strainer set over a large bowl. Strain; discard solids. Repeat with remaining corn and liquid. Stir buttermilk, salt, pepper and 1 cup corn liquid into bowl. Cover and refrigerate until well chilled, 2 hours.
In medium bowl, toss together tomato, vinegar, oil, thyme and reserved 1/2 cup corn to combine. Ladle chilled soup into bowls and top each with 2 tablespoons tomato mixture.
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