Custard Cream Buns (Sa-la-pao Sai Cream)

2 cups all purpose flour½  cup cake flour½  cup sugar1 tsp salt½  tsp ammonia1 tbsp yeast1 cup lukewarm water¼  cup vegetable oilsheets of paper cut into 2"x2"squares, used as a baseFILLING:
5 egg yolks1 cup fresh milk½  cup sweetened condensed milk¼  cup wheat flour2 cups sugar¾  cup butterPREPARATION:
Combine the sweetened condensed milk, fresh milk and sugar together, stirring well. Add wheat flour and stir thoroughly until all lumps are dissolved. Beat the eggs until fluffy, then add the batter and stir to mix well. Transfer to a double-boiler; simmer over low heat until the mixture thickens. Add the butter stirring regularly and simmer until thick and done. Remove from the heat.Mix the yeast, lukewarm water sugar and salt together Stir until all the sugar has dissolved. Mix both kinds of flour with ammonia, pour in the yeast mixture and oil. Knead until a smooth dough is formed. Place the dough in a greased mixing bowl, cover and let stand until it doubles in size.Divide the dough into 20 gram portions, roll into balls. Flatten each ball, and make a dome in the center Spoon I tbsp of the custard filling into the center; gather the sides to seal closely. Place each bun on a sheet of paper and arrange on the dim sum trays. Bring the water in a steamer to a boil; place the dim sum trays in the steamer and steam for 10 minutes. Remove and enjoy while hot.