In a large bowl, combine glutinous rice flour and salt. Gradually add hot water (about half the flour weight, adjust as needed) and knead until a smooth, pliable dough forms. Cover the dough with a damp cloth or plastic wrap and let it rest for 15-20 minutes.

While the dough rests, prepare your fillings. For the sesame filling, mix toasted sesame seeds, sugar, and 0.5 tsp sesame oil in a small bowl. If using red bean or chestnut paste, ensure it's ready to use.

Pinch off small pieces of dough (about walnut-sized) and roll into a ball. Flatten each ball into a disc, creating a small indentation in the center. Place about 1/2-1 teaspoon of your chosen filling into the indentation.

Fold the dough over the filling and carefully seal the edges, shaping it into a half-moon. Ensure there are no cracks so the filling doesn't leak during steaming. Repeat with the remaining dough and filling.

Line a steamer basket with parchment paper or, traditionally, fresh pine needles. Arrange the shaped Songpyeon in the steamer, making sure not to overcrowd them. Ensure the water in the steamer pot is boiling.

Steam the Songpyeon for 15-20 minutes, or until they appear slightly translucent and chewy. Be careful not to overcook, as they can become tough.

Once steamed, carefully remove the Songpyeon from the steamer. Immediately rinse them briefly under cold running water to prevent sticking, then drain well. Brush each Songpyeon lightly with sesame oil to give them a glossy finish and extra flavor.

Serve Songpyeon warm or at room temperature. They can be stored in an airtight container for a day or two, though they are best enjoyed fresh.