Mix the milk, yeast, and sugar together.

Add the all-purpose flour and vegetable oil.

Stir until no liquid remains.

Roughly knead into a dough, cover with plastic wrap, and let rest for 10 minutes.

While the milk dough is resting, make the purple sweet potato dough: In a bowl, combine milk, yeast, sugar, and mashed purple sweet potato. (To make the mashed purple sweet potato: peel, wash, and steam the sweet potato for 30 minutes, then mash while hot. You can make extra and freeze for later use.)

Add the flour and vegetable oil, and mix until no liquid remains.

Roughly knead into a dough, cover with plastic wrap, and let rest for 10 minutes.

Take out the milk dough and knead until smooth and elastic, then shape into a ball and place in a bowl (do not cover with plastic wrap).

Take out the purple sweet potato dough, knead until smooth and elastic, shape into a ball, and place in a bowl covered with plastic wrap.

Weigh the milk dough and divide into 10 pieces, about 1.75–1.8 oz (50–51 g) each. Flatten each piece with your palm.

Fold the edges inward.

Tuck the edges in to form a round ball.

Gently roll each ball, and arrange them in order.

Prepare a little cooking oil and a brush. Roll each dough ball into an oval about 7–7 1/4 inches (17–18 cm) long, and brush the entire surface lightly with oil.

Fold the dough in half from top to bottom.

Use a chopstick to gently press the dough inward at the center of the edge.

Press twice more on each side.

In total, press five spots including the center.

Flip the dough over and place it on parchment paper, then arrange in a steamer basket. Repeat with all 10 pieces, shaping and placing them in the steamer.

Deflate the purple sweet potato dough and roll into a log. Take 1.75 oz (50 g) of the dough and divide into 10 pieces (about 0.18 oz/5 g each). Roll each into a ball, flatten, moisten one side with your finger, and stick it slightly below the center of each bun.

Take the remaining 2.8 oz (80 g) purple sweet potato dough and divide into 40 pieces (about 0.07 oz/2 g each). Roll each into a ball, flatten slightly, moisten with water, and stick them above the center purple sweet potato piece. If the dough is too dry while rolling, moisten your fingers before shaping.

Once all are assembled, cover and let rise for 25–30 minutes.

Start steaming with cold water. Steam for 20 minutes, then gently open the lid a little to let the steam escape before removing the buns to cool. Once cooled, you can store them in a zip-top bag in the freezer. To eat, steam for 8–10 minutes until heated through.