Gluten-Free Sticky Rice Buns

Bring rice (do not rinse rice before cooking to ensure optimal stickiness), salt, vinegar, and 3 cups water to a boil in a medium saucepan. Reduce heat to low, cover, and cook until water has evaporated and rice is cooked, about 20 minutes. Keep covered and let cool until cool enough to handle, about 30 minutes. Stir in scallions and 2 Tbsp. sesame seeds.
Fill a small bowl with room-temperature water. Scoop about 1/2 cup rice mixture with wet hands and form into a ball. Pat rice, dipping your hands in water to keep them wet, into a firm patty about 4" round and 1/2" thick. Wrap tightly in plastic wrap, then repeat with remaining rice mixture until you have 8 patties.
Heat a griddle or large cast-iron skillet over medium-high. Brush 1 side of each patty lightly with sesame oil. Working in batches if needed, cook patties oil side down until lightly browned, about 6 minutes. Brush tops with oil, then flip and cook until other side is lightly browned, about 5 minutes more. Transfer to a wire rack, then sprinkle with sesame seeds.
Rice buns can be formed, not cooked, 1 day ahead; keep wrapped and chill.