Begin one day before by soaking your glutinous rice in cool water at room temperature overnight.
The next day, clean your banana leaves with a damp cloth. Quickly pass each leaf over an open flame to make them soft and bendable, then cut them into 8x6 inch rectangles.
Drain the soaked rice thoroughly in a cheesecloth or strainer. Make sure to squeeze out all excess water.
Put the drained rice in a blender and grind it until it becomes a fine powder.
Add your coconut milk and brown sugar to the rice powder in the blender. Blend everything until smooth and well mixed. Pour this into a bowl and gently stir in the young coconut strips.
Get your charcoal grill hot to medium-high heat (375°F/190°C). While it's heating up, start wrapping your tupig.
Take a prepared banana leaf and put two tablespoons of the mixture in its center. Carefully roll the leaf around the mixture, then fold in the edges to seal it. The mixture will be quite runny, so work carefully.
Place each wrapped tupig on the hot grill right after you wrap it. Cook for about 8-10 minutes total, turning every 2 minutes so it cooks evenly. You'll know it's done when the banana leaves are nicely charred and the tupig feels firm when you gently press it.
Take them off the grill once cooked. Let them cool slightly before eating - just enough so you don't burn yourself. When you open the banana leaf, you should see a golden-brown rice cake that's slightly crispy outside and chewy inside.
Serve while still warm. If you need to save some for later, let them cool completely first, then store in an airtight container. They'll keep at room temperature for two days, in the fridge for up to five days, or in the freezer for one month. To reheat, just put them on a hot grill or pan for 2-3 minutes on each side.