Put the coconut milk and salt in a saucepan, and bring to a boil. After it has started simmering, add the cornstarch diluted in the water, bring to a boil and wait for the sauce to thicken. Take off the heat, and wait till it cools. The sauce should be a teensy bit sweet; if it's not, add 1 or 2 lumps of sugar to taste.
Cut the bananas in half along the horizontal axis. If using large ones, also cut them in half vertically.
In a frying pan, put a fair amount of oil, and panfry the bananas until they turn soft and slightly browner (alternative version, just cut the Cavendish vertically, don't touch the small ones; wrap the bananas in foil and put them in an oven, broiling for a few minutes at 275°C).
Remove and serve, putting the coconut sauce over the bananas and the crushed peanuts.
Recipe by Aliette de Bodard at https://www.aliettedebodard.com/recipes/chuoi-nuong-grilled-bananas-coconut-sauce/