Drop tomatoes and corn into a large pot of boiling, salted water; return to a boil on highest heat. Boil 15 seconds. Remove tomatoes; let corn boil 1 minute, then remove. Cover pot and lower heat.
Peel tomatoes, halve, and squeeze out seeds. Cut one tomato into ½-inch dice. Cut kernels from cob.
Boil spaghettini until just barely tender. Meanwhile, wh