Chinese Steamed Custard Buns (sau bau)

2 ½ oz butter
2 eggs
½ c. sugar
¼ c. custard powder
¼ c. corn starch
1 c. milk
4 c. flour
7-8 tsp. baking powder
1 c. sugar
2 tbsp (1 oz) lard
1 tsp. vinegar
1 c. cold water
24 2 ½" parchment paper or paper muffin cups
Melt butter. Mix in custard powder and corn starch in a separate bowl. Beat eggs and sugar until sugar dissolves. Stir in butter, custard powder and corn starch. Add milk. Steam the mixture over rolling hot water for 15 minutes. Take the filling and stir again. If it’s too lumpy, add more milk until it’s smooth. Return the custard to the rolling hot water and steam for another 15 minutes. Let it cool completely.
Combine the flour and baking powder in a large bowl. Cut the lard into the flour until the mixture resembles fine crumbs. Mix in sugar and vinegar. Gradually add water. Use one hand to mix until it just holds and forms a dough. Add a little more more water if 1 cup is not enough. Leave the dough in the bowl and cover it with a damp towel for half-an-hour. Divide the dough into four portions. Divide each portion again into 6 balls. Use a rolling pin or fingers to flatten each roll into a 3" or 4" round. Stuff in the desired amount of filling and squeeze the opening closed to form a bun. Place on top of parchment paper and then arrange in a steamer for steaming.