In a large stockpot, bring several inches water to a boil over high heat and then add the corn. When the water returns to a boil, cook for another 5 minutes. Remove with tongs and drain off excess water. Remove to a plate and pat dry.
Over an open gas-stove flame char one of the ears of corn, rotating it every so often until slightly blackened on all sides. Remove to a serving platter and repeat with the remaining ears of corn.
Combine the fish sauce, water, palm sugar, salt, chile pepper and scallions in a small bowl and stir until the palm sugar is dissolved.
Heat the vegetable oil in a small saucepan over medium-high heat. Add the fish sauce mixture, bring to a boil and let simmer until the sauce has thickened slightly.
Remove from heat and brush the mixture over the corn. Serve immediately.
Note: Feel free to prepare the corn on the grill or just use steamed corn. Above is the method I use when I want the taste of grilled corn but don't have access to a grill.