COOK THE SAGO PEARLS: Bring 3 cups of water to a boil in a pot. Add the tapioca pearls and boil for 15 to 20 minutes, stirring occasionally to keep them from sticking. Turn off the heat, cover the pot, and let the pearls sit for another 15 minutes until they turn fully translucent. Drain and rinse the pearls under cold water, then set aside.

MAKE THE ARNIBAL SYRUP: In a small saucepan, combine brown sugar and water. Bring the mixture to a boil over medium heat, then reduce the heat and let it simmer. Stir occasionally until the sugar is completely dissolved and the syrup has thickened slightly, about 7 to 10 minutes. Add vanilla extract if using, then remove from heat and let it cool slightly.

PREPARE THE SILKEN TOFU: In a small bowl, dissolve the agar-agar powder in 1/4 cup of water. Pour the soy milk into a saucepan and heat it gently over medium-low heat, making sure it does not boil. Stir in the dissolved agar-agar and continue to stir constantly for 5 to 7 minutes until the mixture starts to thicken slightly. Pour the mixture into serving glasses or ramekins and let it cool and set for about 30 minutes at room temperature, or place it in the refrigerator to speed up the setting.

ASSEMBLE THE TAHO: Once the tofu is set, spoon a layer of warm or room-temperature arnibal syrup over the top, followed by a generous spoonful of sago pearls. Serve immediately while still warm or gently reheated.