Put corn in a 5- to 6-quart wide pot, then add water to cover and bring to a boil, covered.
Remove from heat and let stand, covered, 5 minutes.
Transfer corn with tongs to a cutting board. When cool enough to handle, cut kernels off cobs with a large heavy knife.
Meanwhile, whisk together lemon zest and juice, butter, honey, salt, and pepper in a large bowl until combined.
Wash quinoa in 3 changes of cold water in a bowl, draining in a large sieve each time.
Cook quinoa in a 4- to 5-quart pot of boiling salted water, uncovered, until almost tender, about 10 minutes.
Drain in sieve, then set sieve over same pot with 1 inch of simmering water (water should not touch bottom of sieve). Cover quinoa with a folded kitchen towel, then cover sieve with a lid (don't worry if lid doesn't fit tightly) and steam until quinoa is tender, fluffy, and dry, about 5 minutes.
Remove from heat and let stand (still covered) 5 minutes.
Add quinoa to dressing and toss until dressing is absorbed, then stir in corn, scallions, mint, and salt and pepper to taste.
