Step 1Heat grill to medium-high. Grill corn, turning occasionally, until cooked through and lightly charred, 10 to 12 minutes. (Alternatively, drop corn in a large pot of salted boiling water and cook for 5 to 7 minutes.)Step 2Meanwhile, zest the lime. Set aside the zest then cut the lime in half. Cut one half into four wedges and squeeze the remaining half into a small bowl. Stir the mayonnaise, fish sauce and sugar into the lime juice.Step 3Slather hot corn with mayonnaise mixture. Sprinkle with chopped peanuts, or, for maximum coverage, roll corn in peanuts spread on a plate. Sprinkle with lime zest and cilantro, if using. Serve with lime wedges alongside for squeezing.