My wife found Japanese sweet potato (satsuma-imo) at Whole Foods and had to get them. We like these kind of sweet potatoes because they have firmer flesh than U.S. yams and are very sweet. We made a few of our usual dishes from them. One is cooking them in the Weber grill when we grill chicken. Simply, wash, prick all over with a fork (so they don't explode while cooking), wrap them in aluminum foil and stick them in the Weber at the same time the chicken is put in. By the time, the chicken is done the potato is also done. We grilled 4 sweet potatoes this way one weekend. We ate some of the sweet potato with the chicken for dinner. The next day, my wife made these sweet potato rolls with the leftovers. Although the rolls are based on a recipe she found, as my wife was making these rolls she realized she had lots of extra sweet potato. Since she is particularly fond of rolls with surprise fillings she decided to put the extra sweet potato into the center of the the roll as shown. It turned out to be quite successful.
Although she did not add any sugar to the sweet potato filling, it is really sweet and reminded me of "white anko paste" or "shiro-an" 白あん made of white beans grown in Hokkaido.
We cooked 3 Japanese sweet potatoes wrapped in aluminum foil in the Weber grill when we barbequed chicken one weekend. Let cool, then peeled and thoroughly mashed in a food mill so it is smooth. You should have 1/2 cup of flesh.
Combine the yeast with 1 tablespoon of the sugar and the warm water in the bowl of a stand mixer; proof for 5 minutes. Add 2 eggs and beat on low speed, then add the remaining 3 tablespoons of sugar, the butter and salt. Beat on low speed for about 2 minutes (no need to scrape down the bowl), then add the sweet potato and beat for about 1 minute. Add 1 cup of the flour at a time, beating to form a slightly stiff dough that has pulled away from the sides of the bowl; add flour as needed.
Transfer the dough to a lightly floured work surface; knead for 2 to 3 minutes. When it is smooth and springy, shape it into a ball. Use oil to lightly grease the inside of a large bowl, then place the dough in it, turning it to coat evenly. Cover the bowl with plastic wrap; let the dough rest for about 1 hour or until it has doubled in size. The dough will be ready when you can push 2 fingers into it and the indentations remain.
Punch down the dough. Cut into pieces weighing about 2 oz. each. Flatten the dough (#1) and put a small scoop of the extra, sweet potato (#2) in the middle (#3). Pinch the dough around the sweet potato ball and form into a roll (#4). Place in a greased baking dish. Cover with plastic wrap; let the rolls rise for 45 minutes to 1 hour or until doubled in size. Cook in a preheated oven of 375 degrees for 15 to 20 minutes until golden.
These rolls were wonderful. Very light with a lovely mild sweetness. The center of sweet potato was a really good addition. It was soft and also sweet.