In a 2-quart (2 L) saucepot, combine the coconut milk, 1½ cups (360 ml) water, the kosher salt, and brown sugar and bring up to a boil.
Add the glutinous rice, return to a boil, then reduce to a slow simmer.
Put a lid on the pot and cook for about 25 minutes, occasionally lifting the lid to stir.
Remove the lid and cook for another 5 to 10 minutes, until the rice has absorbed most but not all of the liquid.
If the center of the grain is still quite hard, add a bit more water, ¼ cup (60 ml) at a time, and continue cooking until the rice is al dente.
Preheat the oven to 375°F (190°C) and arrange the racks so one is at the top level and another at the middle.
Line an 8 by 8-inch (20 by 20-cm) pan with the banana leaf, if using.
Transfer the cooked rice to the banana leaf-lined pan.
In a small saucepan over low heat, melt the coco jam into a clear syrup.
Using a spoon, spread the melted jam over the cooked rice in an even layer.
Bake the biko for 25 to 35 minutes on the middle rack of the oven.
Transfer to the top rack and set the oven to broil.
Let broil for 2 to 3 minutes, until the surface of the cake is bubbling and takes on the luster of dark caramel.
Let the biko cool for about 20 minutes.
Cut into 2-inch (5 cm) squares and serve at room temperature with a fillip of flaky sea salt on top.
