Dissolve 葛 in some water, strain unto sugar, cook it all for about 15' on medium heat until it becomes thick and a bit translucent. Normally you now scoop it into the bamboo leaves, but I think it can go into a 流し器 now, be steamed in a cloth lined steamer for 10' on high, and then be cut.
To fill, form a cornetto with the leaves, then pull the part in the back through to come out at the tip of the cornetto.
⚠️ Don't store in the fridge lest they become rubbery. They can, however, be slightly chilled or served on ice before serving.
To eat, push in the leaf tip through the cornetto, then unfold and cut with a 黒文字.