Put sifted flour, custard powder and salt into the bowl of an electric mixer fitted with a dough hook. Mix for 10 seconds. Add yeast and sugar with the lukewarm water and milk. Leave aside for 10 minutes in a warm corner of the kitchen.
Mix in 20g of softened butter and beat for 4-5 minutes or until dough is soft and elastic.
Remove dough to a lightly greased mixing bowl. Rub the dough with a little corn oil then cover and set aside for 50 minutes to an hour or until the dough has risen.
Punch down the dough and knead lightly. Form dough into a long sausage roll and cut into approximately 30 equal pieces.
Flatten the small balls of dough and wrap a small piece of frozen butter placed in the centre of the dough. Add a piece of topping and set aside to proof for about 20 minutes or until the buns double in size.
Bake the buns in a preheated oven at 200°C for 15 minutes or until golden brown. Remove and serve hot.
For the topping: Beat butter, shortening and sugar together until fluffy. Add in the egg gradually and mix well. Tip in bicarbonate of soda, pineapple juice, flour and milk powder. Mix well then divide into 30 equal portions. Shape into a small ball, then roll into a thin round sheet.