In a mixing bowl, combine the glutinous rice flour and water. Stir until a smooth dough forms. If the dough is too sticky, add a little more flour; if it's too dry, add a bit more water.

Divide the dough into small pieces, about the size of a marble. Roll each piece into a ball and set aside.

Bring a large pot of water to a boil. Carefully drop the dumplings into the boiling water. Cook until they float to the surface, about 3-5 minutes.

While the dumplings are cooking, prepare the sweet soy sauce by combining soy sauce, brown sugar, and cornstarch in a small saucepan over medium heat. Stir until the mixture thickens, about 2-3 minutes. Remove from heat.

Once the dumplings are cooked, use a slotted spoon to remove them from the water and transfer them to a bowl. Drizzle the sweet soy sauce over the dumplings and sprinkle with sesame seeds if desired.

Serve warm or at room temperature.