Combine the bread flour and cake flour in a bowl. Add the caster sugar and dry yeast near to you, and add the salt on the far side. Combine 30 g of the egg and milk, then warm up the mixture until it is warm to the touch. Pour it over the yeast.
When it's no longer floury, place the dough onto a counter. Knead the dough for about 10 minutes, pushing it down and out with your palm. When the dough looks glossy, add the margarine and knead some more.
When you add the margarine, the dough will break apart to start with, but it will gradually come together.
Once you have finished kneading, pull the top part of the dough down to the bottom to shape a ball. Seal the bottom and put it back to the bowl.
Cover the bowl with cling film and wet kitchen paper. Let it rise with the oven's proofing function at 40°C for 40 minutes for the first proofing. (In summer, you can let it rise at room temperature.)
Mash the sweet potato with a spoon. I used `anno-imo` and didn't add anything, but if you add some sugar and butter, add at this point.
After the first proofing, divide the dough into 8 portions. Shape each portion into a ball, firmly seal at the bottom. Cover with cling film and wet kitchen paper, and leave them to rest for 15 minutes.
After 15 minutes, roll the dough out to an oval (rectangle) shape. Spread the sweet potato paste on the front 2/3 of the dough.
Roll it up from the front, and seal the edge and both ends firmly.
Place the dough joint-side up, make a long cut in the center with a scraper, leaving space on the both ends.
Take one end and feed it through the center about 3 times.
Shape the dough nicely and place on a baking tray. Cover with cling film and wet kitchen paper. Let it rise with the oven's proofing function at 40°C for 30 minutes for the second proofing. (In summer, you can let it rise at room temperature.)
After the second proofing, preheat the oven to 190°C. Meanwhile, brush the leftover beaten egg on top of the dough, and sprinkle some black sesame seeds.
Bake in the oven for 17 minutes at 190°C. And it's done! (Adjust the cooking temperature/time depending on your oven.)
Story Behind this Recipe
I wanted to make finger rolls out of sweet potato because they are easy for children to eat.
I bake these rolls at slightly high temperature as it is hard to brown them in our oven. Please adjust the baking temperature and time depending on your oven. Check the sweetness of the sweet potato before you add the white caster sugar or butter.