If you have dried out shiitake mushrooms, I suggest soaking them for a whole morning to rehydrate them. You will then remember to dry them well before adding them to your stuffing.
Let's start with the dough: Mix all the dry ingredients, flour, yeast, salt, sugar with a whisk. Make a well in the center and add your warm water. Mix by kneading with your hands, then add the oil. Mix well for at least 5 to 10 minutes to obtain a homogeneous paste. Reserve this dough in a container under plastic film and let it swell at least 20 minutes.
Meanwhile prepare the stuffing: Finely chop your ingredients, meat, chives, mushrooms and add the seasonings salt, pepper, soy sauce, sesame oil, cornstarch, spices ... Mix and knead everything. If you have a food processor you can mix all these ingredients.
Preparation of the nikuman: Once your dough has puffed up, knead it and cut it into 10 small balls. Roll out one of the balls with a rolling pin and insert the filling inside with a tablespoon or an ice cream scoop. Hold your dough and filling in the palm of your hand. Wet the edges with water. Using your other hand, fold the edges into the center, turning with your fingertips. Make sure the bun is closed, don't hesitate to pinch the center.
The nikuman bun is ready, you can steam it by putting a film cooking in the bottom during 10 to 20 minutes. If you don't have a machine, you can make them in the microwave by moistening the nikuman and putting them in a dish with a paper film on the top. You can cook them at 600 watt for 6 minutes.