Bring beans to boil with 3 c water, then time it for 5 minutes
Take off from heat, drain
Place back into the pot, add in 3 c water, bring to boil/simmer in low heat for at least 2 1/2 H (adding water as needed a long the way)
Take off from heat, drain
Place back into pot and add in sugar, cook in medium-high heat for 15 minutes, keep stirring
Place in container, and cool to room temperature
Use once cooled immediately or can be stored in the fridge
Dough
Dissolve yeast in warm water, set it aside for a few minutes till bubbly. Then add the whisked egg, butter chunks and sugar. Sprinkle the salt over the top and add the flour, cup by cup till incorporated. Set aside, loosely covered on the counter for a couple of hours till risen. Use immediately.
Punch lightly risen dough, divide into equals balls. Flatten with hands or rolling pin, place at least a tbsp of filling in the center, seam to seal.
Place buns on individual parchment paper or cupcake liner, and paint as you wish.
Bring water to boil and place bamboo steamer into a pot.
Steam buns for 7 minutes.
Notes
During cooking with sugar, I mashed the beans with the wooden spoon and occasionally with potato masher (as I like my paste to be smooth or just somewhat chunky)