To make the tsubu-an, refer to. I partially thawed a stock I had in my freezer. https://cookpad.com/en/recipes/143833-homemade-tsubu-an-chunky-sweet-azuki-bean-paste
Sift together the cake flour and baking soda, add the sugar, then mix thoroughly with a whisk.
Add the beaten egg and sake to the mixture from Step 2. Adding a little water at a time, knead until the mixture forms a dough about the firmness of your ear lobe. Form into a ball.
With a tightly wrung out kitchen towel, wrap the dough, and let it sit for about 30 minutes.
Stretch the dough out into a log, then cut it into 10 even pieces with a pastry scraper or knife. Until you wrap them around the anko, keep moist under the kitchen towel. Do not let them dry out.
Roll out the individual pieces of dough into roughly 10 cm ovals, place the tsubu-an in the middle and wrap. Lightly press the surface of the dough into shape.
With the seam facing down, place the manju onto muffin cups.
Prepare a steamer, and once it starts to steam, place the manju inside. Steam for about 15 minutes. This is how they should look after steaming.
The tsubu-an wasn't quite centered in these...
In regard to the amount of baking soda, some require 3 to 4 g per 100 g of flour, and some only 1 to 1.5 g.
The baking soda I use says to use 6 to 8 g per 200 g of flour, but it becomes bitter if you put too much.
When I used 4 g, the manju rose sufficiently.