Prepare the Dough: In a large mixing bowl, combine the flour, sugar, salt, and yeast. Mix the dry ingredients well.
Add the water and vegetable oil to the yeast mixture. Use a dough hook attachment with your stand mixer to mix the ingredients on low speed until they come together and form a ball of dough.
Increase the speed to medium and knead the dough for 5-7 minutes until it becomes smooth and elastic. Cover the dough with a damp cloth and let it rest for 1 hour or until it doubles in size.
Prepare the Filling: In a saucepan, combine the red beans, sugar, cinnamon, and butter. Cook the mixture over low heat for 10-15 minutes, stirring frequently, until it thickens and becomes a smooth paste.
Divide the dough into 8 equal parts. Flatten each part with your hands or rolling pin on a smooth surface and place a tablespoon of the sweet red bean paste filling in the middle of the dough.
Gather the edges of the dough, and pinch them together to seal the bun. Place the smooth balls, sealed side down, on a baking tray lined with parchment paper. Repeat the process for the remaining dough and filling.
Place the filled dough balls on a piece of parchment paper, and cover with a damp towel. Let them rise for another 20-30 minutes in a warm place. Heat a steamer over medium heat.
Once the water is boiling, place the buns in the steamer basket, leaving space between them to expand. Steam the buns for 10-12 minutes, until they are puffed up and cooked through.
Serve the fluffy buns warm