Place the flour, instant yeast, sugar in a mixing bowl. If you are using a stand mixer, use a dough hook attachment. Add cold milk or room temperature water. Add cooking oil. You may need to add more milk/water if the dough is still a bit dry. Add more teaspoon by teaspoon until the dough comes together and continue to knead for 10-12 minutes until the dough is very stretchy and smooth
Make the dough by hands:
Place the flour, instant yeast, sugar in a mixing bowl. Mix until you get a rough dough (it's not going to be smooth yet). Cover and rest it for 15 minutes. Then go back and knead it. You'll be surprised by how easier it is to knead it now. Knead until the dough is smooth and pliable, about 15-20 minutes. At any point during kneading, you can always stop and rest the dough if you find it hard to knead. This is to relax the gluten. Then go back to knead again and you'll be surprised by the difference it makes by just resting the dough before kneading again. It's easier on you too.
Rest the dough (not supposed to double in size):
Wrap the dough with a plastic wrap and let it rest for 15 minutes before shaping. This is to relax the gluten, not to proof the dough. Pinch off about 15 gr of dough for the rat's ears and tails and 5 gr for the nose. So now you have 1 large dough and 2 smaller ones for ears and noses
Color the ears and noses:
Divide the large dough into 12 equal portions, cover with a plastic wrap to prevent from drying out. Drop about 1 drop of orange gel food color to a 15 gr dough and then knead it to distribute the color. Divide into 24 equal balls for ears and 12 smaller portion for the tails. Drop a tiny drop of red gel food color to the 5 gr dough and then knead it to distribute the color. Divide into 12 tiny pieces.
Shaping:
Flatten the large orange balls to shape like ears and roll the smaller orange dough between your palms to shape into tails. Cover them with plastic wrap to prevent from drying out. Lightly dust the work surface with some flour. Flatten the large white dough with your palm and then use a rolling pin to roll it out to form a circle about 4-5 inches, with a slightly thicker middle part. This is to support the weight of the filling. Don't roll the dough too thin. If you roll the dough too thin, the filling might make the steamed buns "wrinkle" later when you steam. Place the filling at the center and then gather the sides to enclose the bun and pinch to seal. Flip the bun so the seam side is down now
Cup the dough with both palms and move the dough in a circular motion to shape it taller. This part is important so your steamed buns come out tall instead of spreading to the side after steaming. Then shape it into a slightly oval shape. Repeat with the rest of the dough. Cover with a clean kitchen towel
Dab the part of the dough where you will attach the ears and tails with a bit of water and gently attach the ears and tails there. Do the same for the nose and the eyes. The water helps them stick to the large white dough. Repeat with the rest
Proofing:
Cover the shaped rat buns with clean kitchen towel. Let the dough rise at room temperature. If it's winter where you are, you can use your oven "bread proof" function to let them proof in there, or simply turn on your oven to lowest temperature and then turn off and after 15 minutes, place the shaped buns in there to let them proof. They won't necessarily double in size, but at least puff up to about 50% of original size. This may take about 10-15 mins at warm temperature. Do not overproof your dough
Most likely your steamer won't be able to accommodate steaming all buns at one go. You may need to steam in 2-3 batches, which means, the rest of the batches will sit longer and continue to proof while waiting for the steamer. Not good for the buns! Here's what you can do: Make sure the cover them with plastic wrap and then place them in the refrigerator to slow down the yeast activity or halt it all together until they are ready to be steamed
Steaming:
Bring the water in your steamer to a rolling boil. Wrap the lid of your steamer with a kitchen towel to prevent moisture dripping back on the steamed buns as this will create "burn" spots
LOWER THE HEAT TO MEDIUM. Place the buns in there, leaving about 1-inch space in between. Cover with a lid but leaving it about 1/4-inch gap for some steam to escape. Steam on medium heat for 15 minutes. This is especially important if you see that you have overproof the steamed buns (the steamed buns have doubled in size and they have widened to the side). As long as you steam them like this, the buns will not wrinkle and collapse later when you take them out from the steamer
After steaming, turn off the heat. DO NOT OPEN THE LID of the steamer. Let the buns sit there for 5 minutes like this. The buns will not sink or wrinkle due to the sudden change in temperature
Cooling:
Remove the buns to a wire rack to let them cool down. This is to ensure the bottom of the steamed buns will not be wet and soggy
Notes
1. You can use active dry yeast, but make sure to rehydrate the yeast in water or milk with 1 tsp of sugar. It will dissolves and be all foamy, about 10 minutes. If it's not, your yeast is not longer good. You can add the rest of the sugar into the dough later