Combine 30g of brown sugar and 2.5g of high-sugar yeast into 130g of water. Stir until the sugar and yeast are fully dissolved. This step helps activate the yeast, which is crucial for better fermentation of the dough.

Slowly pour the yeast water into 250g of all-purpose flour while stirring. Continue mixing until the flour forms a rough texture. Then, knead the mixture into a dough. The dough doesn't need to be perfectly smooth at this stage. Place it in a container, cover with a damp cloth or plastic wrap, and let it rest at room temperature for 20 minutes. Resting allows the flour to fully absorb the water, making it easier to knead into a smooth dough and improving its elasticity.

While the dough is resting, mix 50g of brown sugar and 10g of cinnamon powder in a bowl. Stir well to ensure they are evenly combined. Set this mixture aside.

After resting, take the dough out and place it on a work surface. Knead it for a few minutes until it becomes smoother. If you prefer a softer, fluffier bun, you can knead it longer until the dough is smooth and elastic. Divide the dough into 5 equal portions using a knife.

Take each small dough portion and fold it inwards, making it into a ball. Roll each ball into a tongue-shaped flat dough piece using a rolling pin. Evenly sprinkle a layer of the brown sugar cinnamon mixture on the dough. Start rolling from one end to form a cylinder, pinching the ends tightly to seal in the filling. Then, cut the cylinder in half and twist the two pieces together, forming a spiral shape. Tuck both ends to seal. Place parchment paper under each cinnamon sugar bun. Repeat with the remaining dough to make more buns.

Arrange the prepared buns on a steamer tray, making sure to leave some space between each one to prevent them from sticking together during fermentation. Place the tray in a warm environment (around 30°C) to ferment for 40-50 minutes. The buns should expand noticeably. Press gently on the surface of the dough; it should slowly bounce back.

Once the buns have fermented, place them in a steamer with cold water. Cover and bring the water to a boil over high heat. Once boiling, reduce the heat to medium and steam for 15 minutes. After 15 minutes, turn off the heat but leave the lid on for 3 more minutes to prevent the buns from shrinking due to sudden temperature changes, keeping them soft.

After the resting time, open the lid, remove the cinnamon sugar buns, and let them cool slightly on a wire rack before serving. Enjoy!