This is not quite an authentic recipe, I think. At least the sausage buns I've seen have also had egg custard or were a bun wrapped around a whole sausage. Also, I doubled the scallions. There's never enough in the traditional ones.
I worked off of the recipe from here. I made some minor adjustments for adding the sausage and skipped the fancy styling, though.
For the dough:
1 1/2 teaspoons yeast
3 cups bread flour
2 oz sugar [down from three in the original recipe]
1/4 cup vegetable oil
1 cup milk
For the filling:
6 Tablespoons chopped scallions, green part only
1 link lop chong, steamed for five minutes or microwaved for one to partially cook and then sliced thin and chopped
1 teaspoon salt
1/2 teaspoon sugar
1 tablespoon sesame oil [double if you're not using the sausage]
0. Turn on oven to lowest setting.
1. Mix flour, sugar and yeast (or activate yeast in milk if you're using that sort). Mix in vegetable oil and milk until a slightly stiff dough forms. Let rest 20 minutes. Knead for five minutes until gluten forms. Put in clean bowl, cover with moist cloth and put in oven for 1 hour.
2. Mix filling ingredients and allow to macerate.
3. After an hour, remove dough from oven and scrape out onto a floured work surface. Roll dough out to form a 14" by 20" rectangle. A little thinner would probably be fine but that's how big my cutting board is so that's as far as I got. Spread filling over dough. [I used 4 Tablespoons of scallion to start but decided it was a bit sparse so I chopped up a bit more. Another half a sausage wouldn't be a bad either.
4. Roll the dough up along the short axis and slice into two inch pieces. Use a sharp knife and and sawing motion with little downward pressure to keep from squishing the dough. Stand the slices up on baking sheets with plenty of room around each and return them to the oven that I never told you to turn off. Let rise 40 minutes.
5. Start a steamer steaming. When buns are well risen, move to steamer and steam 13 minutes. They'll expand again so don't crowd the steamer. I did batches of four but that only worked because my end pieces rather small. If you did step 3 better than I did, you'll have to do batches of three.
And that's it! Serve immediately with a completely optional soy/chili oil dip or cool and freeze. Reheat 60 seconds in the microwave covered with a damp paper towel.
These turned out very well. I'll put them up against the best I've had in various Chinatowns. The dough is just as it should be: fluffy and light, but squishy and chewy to the bite. Mild, but not flavorless. A lot of recipes didn't use milk, but I quite like the character it added. The scallions and sausage are aromatic and brightly savory, cutting through the sweetness of the buns. Darn good stuff, but I've got to stop eating them so I can freeze some for later. So now I've made gyoza, sticky rice and buns. I think roast pork is up next in my Chinese snack agenda.