Step 1: Prepare the rice flour slurry. In a medium bowl, combine the rice flour with a small amount of water until a smooth, lump-free paste forms. This prevents lumps when added to the hot liquid.
Step 2: Infuse the coconut milk. In a medium saucepan, combine the thick coconut milk, water, and salt. Add the knotted pandan leaves. Bring this mixture to a gentle simmer over medium heat, stirring occasionally. Be careful not to boil it vigorously, as coconut milk can curdle. This step allows the pandan to release its beautiful fragrance, a signature of Indonesian Sweet Coconut Rice Pudding (Bubur Sumsum).
Step 3: Thicken the pudding. Once simmering, gradually whisk in the rice flour slurry. Continue whisking constantly to prevent lumps from forming. The mixture will begin to thicken quickly. Reduce the heat to low and continue stirring for another 5-7 minutes, until the mixture is thick, smooth, and coats the back of a spoon. Remove the pandan leaves. This creamy consistency is what makes Indonesian Sweet Coconut Rice Pudding (Bubur Sumsum) so delightful.
Step 4: Let it rest. Pour the bubur into individual serving bowls or a larger dish. Allow it to cool slightly at room temperature, then chill in the refrigerator until firm, typically for at least 1-2 hours. You can even prepare this component ahead of time and store it in an airtight container.
Step 5: Make the palm sugar syrup. In a small saucepan, combine the palm sugar (gula melaka), water, and a pinch of salt. Bring to a boil over medium heat, stirring until the sugar is completely dissolved and the syrup has thickened slightly, about 5-7 minutes. Strain the syrup through a fine-mesh sieve to remove any impurities. Let it cool completely. This rich syrup is the heart of the sweetness in Indonesian Sweet Coconut Rice Pudding (Bubur Sumsum).
Step 6: Serve. Ladle the cooled Indonesian Sweet Coconut Rice Pudding (Bubur Sumsum) into bowls. Drizzle generously with the cooled gula melaka syrup. For an extra touch, you can add a thin stream of fresh, thick coconut milk or a sprinkle of toasted coconut flakes. Enjoy this comforting treat!