Sift flour, custard powder and baking powder into a large mixing bowl. Sift in icing sugar to mix. Add egg. Combine shortening and butter into the mixture and lightly mix in to combine (do not knead) until a soft dough is formed. Leave to rest for 30 minutes.
Divide dough into portions of 60g each, and flatten into disks. Wrap a piece of filling in the centre and shape into a dome. Arrange on greased baking trays.
Bake in a preheated oven at 180°C for 10-15 minutes. Remove the tray and leave aside to rest for 5 minutes. Brush the mooncakes with egg glaze and continue to bake for another 15 minutes.
For the filling: Combine black sesame paste, peanut butter, salt and butter together. Make into very small balls of 25g each. Divide lotus seed paste into 75g each. Wrap a piece of black sesame paste mixture in the centre of the lotus seed paste ball.