Heat a grill to medium. Pull down the husks of the corn and remove the silk from each ear. Fold the husks back into place and tie the ends together with kitchen string. Soak in a large bowl of cold water with 1tablespoon salt, 10 minutes.
Remove the corn from the water and shake off the excess. Place on the grill, close the cover and grill, turning, until the kernels are tender, about 12 minutes. Remove from the grill and let cool slightly, then remove the husks; brush the corn with 2 tablespoons canola oil and season with salt and pepper. Increase the grill heat to high; return the corn to the grill and cook, turning, until charred, about 5 minutes. Let cool slightly, then cut off the kernels: Stand the corn upright on its stem end and cut down the side of the cob with a sharp paring knife.
Place the avocados in a medium bowl and mash slightly with a fork. Add the corn kernels, serrano chiles, red onion, lime juice, the remaining 2 tablespoons canola oil, the cilantro, and salt and pepper to taste; gently stir to combine.
To keep guacamole from browning, press plastic wrap directly onto the surface and refrigerate until serving.
Photograph by Ryan Liebe
How to remove corn kernels from cob: To remove kernels from cobs of either raw or cooked corn, stand cob upright on its stem end in a large pan, holding tip with fingers. Cut down the sides of cob with sharp paring knife, releasing kernels without cutting into cob. Run dull edge of knife down the cob to release any remaining corn and liquid.
Courtesy of Food Network Magazine