To make the bao buns:

1. Stir together 1/4 cup warm water with yeast and pinch of sugar. Let stand until foamy, 5 to 10 minutes. (If mixture doesn’t foam, start over with new yeast.) Whisk in dried milk and remaining 3/4 cup warm water.
2. Stir together flour and remaining 3 tablespoons sugar in a bowl, then stir in yeast mixture (do not add baking powder yet) with a fork until a dough forms. Knead dough with your hands in bowl until all of flour is incorporated. Turn out dough onto a floured surface and knead, dusting surface and hands with just enough flour to keep dough from sticking, until dough is elastic and smooth but still soft, about 5 minutes. Form dough into a ball.
3. Put dough in an oiled large bowl and turn to coat. Cover with plastic wrap and let dough rise in a draft-free place at warm room temperature until doubled, about 2 hours.
4. Punch down dough, then transfer to a lightly floured surface and flatten slightly into a disk. Sprinkle baking powder over center of dough, then gather edges of dough and pinch to seal in baking powder. Knead dough with just enough flour to keep dough from sticking until baking powder is incorporated, about 5 minutes. Return dough to bowl and cover with plastic wrap, then let dough stand 30 minutes.
5. Cut 16 (3- by 2-inch) pieces of wax paper.
6. Form dough into a 16-inch-long log. Cut into 16 equal pieces, then lightly dust with flour and loosely cover with plastic wrap. Roll out 1 piece of dough into a 6- by 3-inch oval, lightly dusting surface, your hands, and rolling pin. Pat oval between your palms to remove excess flour, then brush half of oval lightly with oil and fold in half crosswise (do not pinch). Place bun on a piece of wax paper on a large baking sheet and cover loosely with plastic wrap. Make more buns with remaining dough, then let stand, loosely covered, until slightly raised, about 30 minutes.
7. Set a large steamer rack inside skillet (or wok) and add enough water to reach within 1/2 inch of bottom of rack, and then bring to a boil. Carefully place 5 to 7 buns (still on wax paper) in steamer rack (do not let buns touch). Cover tightly and steam over high heat until buns are puffed and cooked through, about 3 minutes. Transfer buns to a plate with tongs, then discard wax paper and wrap buns in kitchen towels (not terry cloth) to keep warm. Steam remaining buns in 2 batches, adding boiling-hot water to skillet as needed.

To make the spicy stir fry beef:

1. Put the beef in a bowl and toss in the cornflour and five-spice. Heat the oil in a wok or large frying pan until hot, then add the beef and fry until golden and crisp. Scoop out the beef and drain on kitchen paper. Pour away all but 1 tbsp oil.
2. Add the pepper, half the chilli, the white ends of the spring onions, garlic and ginger to the pan. Stir-fry for 3 minutes to soften, but don’t let the garlic and ginger burn. Mix the vinegar, soy, chilli sauce and ketchup in a Bowl with 2 tbsp water, then pour over the veg. Bubble for 2 minutes, then add the beef back to the pan and toss well to coat. Serve the beef in a soft bun or over rice and enjoy!