In a food processor, blend the red bean and water into a fine mixture. Pour into a pot and add in sugar and salt. Cook on medium heat until the mixture thickens, around 25 minutes. Reduce the heat as it thickens to avoid the bubbling mixture splash out on you, be careful.
Transfer to a plate and cover with cling film. Set aside and let it cool completely before using.
Once cooled, divide the paste evenly into 8 portions.
Add in honey into warm milk and stir until honey is dissolved. Then sprinkle the dry yeast and mix. Leave for it to get foamy, around 3-5 minutes.
In a standing mixer, add in all of the buns ingredients including previously prepared yeast mixture. Mix to incorporate and then knead for 10 minutes. Transfer dough to an oiled bowl and coat the dough with some oil to avoid crust forming on it.
Cover the bowl tightly with cling film. Leave for the dough to rise until double its size in a warm and draft free place.
Once doubled its size, transfer it to a floured working surface. Lightly knead it into a log and then cut it into 8 equal pieces. Form each piece into a ball.
Take 1 ball and roll it into a 3.5 inch (9 cm) circle, more or less. Then place 1 portion of the red bean paste in the middle. Pinch the seams and form it into a ball.
Now roll it into an oval of 4x7 inch (10x18 cm), more or less. Using a sharp knife, make a few straight line cuts on the top dough only, taking care not to cut until the dough at the bottom.
Flip it and start rolling into a caterpillar shape and bend a bit the 2 ends inwards. Place it on a baking tray sprinkled with some flour. Do the same with the rest of the balls.
Brush the buns with some egg wash (beat 1 egg with a splash of milk) and then sprinkle some sesame seeds on top.
Bake in a preheated oven at 340ºF (170ºC) for 15 minutes or until golden brown and fully cooked. Remove from the oven and immediately brush them with some melted butter. Cool completely on a rack.Enjoy!