Prep the corn cobs: remove the corn husks and silk threads, then wash the corn cobs, and pat them dry well with paper kitchen towels, and set aside to completely dry.
Make the sauce: in a small mixing bowl, combine the coconut milk, two tablespoons of fish sauce, teaspoon of sugar, ¼ teaspoon of salt if using it, and 50 g sliced spring onions (scallions). Taste and adjust if needed, adding more fish sauce, sugar or salt to your liking, then set aside. Note: as the flavours meld together, they'll become stronger and more complex, so don’t over-do the salt/sugar.
Cook the corn cobs: lay the four corn cobs down on a dry skillet pan or a grill over medium-high heat (if you’re able to control the heat), then using a silicon brush, dip it into a neutral cooking oil and brush each side of the corn.
Turn the cobs every couple of minutes to ensure the corn cooks evenly; once you’ve turned a cob completely, brush the top side of each corn cob with the coconut milk fish sauce and leave for a minute or two, then turn the cobs, brushing the top side again, leave for a couple of minutes, and repeat for about 15 minutes until all sides are lightly charred and the corn is tender.
Serve the corn cobs: when cooked to your liking, transfer the corn cobs to a serving tray or plate. Brush more sauce over the cobs, sprinkle some fresh slices of spring onions on top, and serve with the remaining sauce in a dish – along with small spoons, a finger bowl and napkins, so guests can help themselves.